Before you leave....
It's always a good thing to prepare before going anywhere, especially when going backpacking to another country for a long period. There is also a lot of information out there that gives you advice and what you need for New Zealand, but I will try to sum up everything you need on here.
First off and a slightly obvious one (excuse the sarcasm) is a flight ticket. These generally cost around £700 - 900 from a fairly good airline. Sometimes you can get a really good deal on websites such as http://flightchecker.moneysavingsexpert.com, www.dealchecker.com and www.statravel.co.uk. I got mine from www.lastminute.com for £830 which I thought was a pretty reasonable deal.
Travel insurance is a necessity too and the price varies depending what your activities and length of stay will be. Again STA travel is a good place for insurance but for a tailored and specific travel insurance for New Zealand travellers http://www.orbitprotect.com is excellent. I'm not sure what the costs are with this company as I get free travel insurance with my Smilemore account. So checking out what your bank offers is always a good idea! Orbit Project is also convenient as you can deal with claims within the country which makes things a lot easier!
Unless your staying for less than a month and not doing any working including voluntary work you don't need a visa. If however you plan on staying for longer and do intend to do those thing you must apply for a working holiday visa. This can last for 12 or 23 months depending on how long you are staying. If you stay for more than 23 months you will need a medical examination fee that can cost around £130 but varies depending on what medical centre you go to. You can find a list of New Zealand government approved Dr's on the official immigration website http://www.immigration.govt.nz/migrant/stream/work/workingholiday.
If you plan on working in New Zealand than opening a bank account is essential as employers won't pay you in an international account. This is extremely easy to do and can be done online and before you go! It usually takes a few days to open the account where you will be given details of your new account so that you can transfer money to it before you leave. When you arrive, all you need to do is pop into a local bank with your identification and receive your card on the spot. They usually require you to show two forms of identification which can be your passport, birth certificate and international drivers license. I opened one with both ANZ and Westpac, just incase I lose a card so I can have another account to fall back on. A list of good banks that you can open an account with are;
ASB - www.asbbank.co.nz
Westpac - www.westpac.co.nz
ANZ National Bank - www.nz.anz.com
Bank of New Zealand - www.bnz.co.nz
Kiwibank - www.kiwibank.co.nz
HSBC - www.hsbc.co.nz
Another essential is an IRD number but this is super easy as well and can be done when you get there. This link http://www.backpackerboard.co.nz/work_jobs/inland_revenue.php provides you will all the information on how to get one.
Speaking of www.backpackerboard.co.nz this website is the online holy grail for people travelling in New Zealand. It gives you so much information and job postings are posted on there daily as well!
Lastly, you should get your vaccinations before you go. As it isn't a high risk country, you shouldn't need too much but your Doctors should be able to advise you on what you need.
Accommodation
There's plenty of accommodation in New Zealand with rooms costing around £10-15 per night for dorm and about £25 for private. But if you get a discount card from Nomads you can save a fair bit on it!
Budgeting
Advice from other website suggest that you should budget around £20 - £25 a day to cover accommodation, food and travel costs if your on a tight budget. But if you've got a bit more cash to spend than £50 a day would be a very comfortable amount to live on.
Getting around
New Zealand has a good travel network and you can get in and around using buses, coaches, trains and inland flights. Though the cheapest option is usually the buses and coaches.