Well, I can't answer that question. People hike the trail in wildly different ways, on wildly different budgets. Some people spend more than $10,000, and we met people that tried to do it on $8 per day (not sure if they ended up meeting that budget or not). There doesn't seem to be a commonly accepted "average amount" for the TA yet, but if I had to guess, I'd put it around 6,000 NZD per person, not including airfare or gearing up.
I do know that one of the most common reasons for abandoning a thru hike is running out of money. It's easy to convince yourself that you can get by on some ridiculously small amount of money - after all, you're living in the wilderness, eating oatmeal and pasta every day, how much could it possibly cost?
Well, a lot more than you think. Over 3,000 km, gear breaks down, willpower erodes, and stomachs get hungrier.
We busted our budget, partly because NZ is more expensive than we expected, and partly because there were some unexpected costs. I'll do my best to give you a starting point for your own budget, and highlight the most unexpected expenses we hit.
(All dollar amounts are in NZD, unless otherwise noted)
Food in NZ is expensive. You'll hear this a lot, and that's because it's true.
We budgeted $10 per person, per day for trail food. This turned out to be a reasonable amount, but became harder to stick to toward the end as our appetites increased and our tolerance for eating the same foods every day wore down. We spent more like $15 pp/day over the last month or so on the trail, and we were eating fairly well, but not buying food indiscriminately.
Prepared food in towns is even more expensive. This is a huge discretionary purchase though. We were willing to pay for prepared meals occasionally, but if you stay strong, avoid buying too many coffees, and cook your own meals in towns, you can keep this expense as low as you want. But don't underestimate the temptation of a soda ($3+), coffee ($4.50), hamburger ($8+), or huge indian meal ($18) when you hit a town.
The amount of money we spent on lodging was one of the bigger, more unpleasant surprises for us. I originally budgeted $50/night (for 2 people), every 6th night, for lodging. My reasoning was that we'd be camping in the backcountry most nights, would stay in campgrounds for less than $20 on many nights near towns, and occasionally pay more for a real bed.
It turned out much differently.
First, camping. On the north island especially, there are many places where there is no free camping available. Either you're road walking past private land, or you're crossing private land where camping isn't allowed, or you're between short bush sections that literally have no flat spots. So, you are often forced to either stealth camp (see below for more on this), knock on a stranger's door and ask to camp on their property, or pay for a campground/hostel. The upshot of all this is that you will either need to get creative while looking for campsites, or expect to pay for lodging more often.
The situation on the south island is much better, mostly due to the awesome hut system, and we only had trouble finding free camp sites for a handful of days near the end. The 6-month hut pass is $90 and is the best deal on the planet, as it buys you lodging for 30+ nights on the trail.
On top of this, campgrounds in NZ, usually referred to as holiday parks, are outrageously priced. They ranged between $15 and $20 per person, per night. Unlike campgrounds in the US, you are charged per person, not per site. We gave up on these very quickly - they are not worth the money. For an extra $10 we could usually find a cheap hostel dorm room. As the trail wore on, and we wore down, we became more willing to pay for a bed, and towards the end we were staying in a private double hostel room whenever we went through a town. Hostels run around $25 per person for a dorm room, and $55-75 for a private room.
Lodging expenses vary hugely from person to person, probably more than any other type of expenditure. Some people get a nice room with private bathroom at every chance, and other people can count on one hand the number of times they paid to stay somewhere. If you are willing to ask to camp on lawns, and get creative, you can keep this expense down. Don't underestimate your desire for a real, dry bed after you've been out on the trail for 4 months. Either way, you still probably need to budget a bit more than you expect here, especially compared to long trails in the US, where free camping near the trail is almost always easy and legal.
Some tips on saving money on lodging:
Friendly kiwi tip: Kiwis are the friendliest people on the planet. If you are in a rural area, don't be afraid to knock on a door if you need a place to set up a tent. More often than not, you will find a place for your tent, but only after you are invited in for tea. We were nervous about this at first, but slowly got over it as we realized that 99% of the kiwis you meet really want to meet you, help you out, and then keep in touch with you after you leave. When looking for a place to camp, we would usually ask "Is there anywhere in the area where we can set up a tent for the night?" This phrasing allows an easy way out if they don't want to host you ("Oh, try the park down the street"), and also makes it a little more clear that you are on foot and not looking for somewhere to park a campervan.
Hostel tip #1: become a YHA member, and get a letter from them certifying that you are a "low carbon traveller" for a huge discount (25% off member prices). This makes YHA hostels a great deal. There are not that many YHA hostels near the trail on the north island, but there is one in almost every trail town on the south island.
Hostel tip #2: Many hostels will let you camp on their lawn for less than a holiday park would charge. Don't be afraid to ask, it's not always advertised.
Stealth camping: As you travel in NZ, you'll hear and see a lot of references to "freedom camping," which is defined as camping on public land for free. Freedom camping was recently made illegal for non-self-contained vehicles (which includes people without vehicles), and now you can technically only camp where signage allows it. It took us a while to figure out why some kiwis had such an opposition to freedom camping - it goes against their super friendly, outdoorsy national identity - but we eventually realized that it all came down to trash and poop. Freedom campers were often inconsiderate, and left piles of trash and poop behind, which, unsurprisingly, upset the locals. My take on this whole thing is that, if you choose a non-obvious spot, leave no trace (especially poop), and generally respect the area you are in, you will have no problems stealth camping when you need to.
This was another surprise for us. We spent quite a bit getting around - around 15% of our entire budget! However, this is an unnaturally high amount for the TA, as it includes bus rides while we were sidelined with an injury, a trip to Stewart Island (expensive!), and a side trip to the Routeburn Track and Milford Sound. But transport can still add up - here's a quick list of trail-related transport that we paid for:
Shuttle to & from Auckland airport: $16 each way
Bus from Auckland to Kaitaia: $55
Bus tour from Kaitaia to Cape Reinga: $50
Water taxi across Waikare Inlet to Russell: $100 (for up to 5 people)
Kayaking the Puhoi River: $50
Canoeing the Whanganui: $250
Crossing the Cook Strait: $75
Water taxi to Queen Charlotte Track: $50
Ride around Rakaia River: $100 (up to 2 people)
Shuttle around Lake Wakitipu: $50
Returning to Auckland from Invercargill: $150+ (via bus to Queenstown/Dunedin, then a flight)
Now, almost of these are optional in some sense, but the alternatives range in difficulty. The Puhoi kayak is only 7km, and many people hitchhike around it to avoid the disproportionate expense. The bus tour to get from Kaitaia to Cape Reinga can also be replaced with a medium hard hitch. The water taxi across the Waikare Inlet can be replaced with a $1 ferry ride and 25km road walk. The ride around the Rakaia can be replaced by 2 hitches - one easy, and one hard enough that you should plan to walk most of the 35km without even seeing a car.
The more willing you are to hitchhike (which we found to be generally easy, safe, and very accepted in NZ), the more you can save here. So, you do have some control, but we found that most of the alternatives were unpleasant enough to justify the cost. If we did it again, the 2 I would skip would be the tour bus to Cape Reinga and the water taxi across the Waikare - we talked to multiple people that hitched these sections without much trouble. The ride around the Rakaia hurt my wallet, but after seeing how empty it was on the south side of the river, I was glad we paid for it.
There were a few other buckets that we spent significant amounts on.
The first was phone and internet. If you're from the US, getting a cell phone plan in NZ will seem like a dream come true. You walk into a Telecom or Vodafone store, pay $5 for a SIM card, and put $20 in your account. You can pay as you go, and top up from just about anywhere. There are a ton of different plans and rates to choose from, but $20/month gave us a reasonable number of minutes (100) and data (500MB).
Wifi is almost never free, even in hostels. Some cafes offer "free" wifi, but generally it's only a tiny amount of data. Expect to pay $5/day whenever you want wifi in a town. The main reason this ended up being a real expense for us was actually this blog - uploading pictures used up too much of our 500MB/month quota, so whenever I wrote posts (almost every town), I needed wifi. Many towns have libraries with free internet, and I took advantage of that whenever possible, but we still ended up spending quite a bit on wifi. (Which really kills me, because, man, the internet is slow in NZ)
The other major expense was, surprisingly, postage. We mailed a bounce box around containing extra shoes, clothes, contact lenses, etc. We also mailed 4 food boxes from Wellington to remote areas on the South Island. All told, we spent over $400 on postage! However, this ended up being worth it...
We went through 3 pairs of shoes each. And shoes are expensive in NZ. Liz got 3 pairs of her shoes, Salomon XA-3D Pros, for less than $100 USD ($117 NZD) per pair before we left. We saw them in NZ stores for $225+ ($299 in one store!) While postage on the bounce box was expensive, our shoe replacements more than made up for it.
Other than shoes, we were very lucky with gear, and replacing it was not a major expense for us. It all held up surprisingly well, and we didn't have to replace anything significant. However, we ran into many people that had to buy new tents, new clothes, new packs, etc along the way. Make sure you budget for gear replacement, especially if you are bringing gear you've never used before - you might hate it!
All told, our expenses broke down roughly like this: 20% trail food, 20% lodging, 15% restaurants, 15% transport, 20% cash (which was probably mostly distributed among trail food, restaurants, and lodging), 10% other (postage, internet, pharmacy, etc). If you are interested in absolute numbers, just shoot me an email.
You can certainly do this trail for less than my guess at the average - $6,000. You can also easily spend more. I know I was frustrated with the lack of resources on budgeting when we were planning for the trail - hopefully this gives you a starting point for your own estimation. Please let me know if you have any questions!