I am currently saving to spent 12 months travelling around NZ. What I would like to know is, when is the cheapest time to fly London to Auckland? I don't mind when the flights are, I just need to know the cheapest. Any help/tips/advice greatly appreciated!!
Also, is it best to get a return then pay to have the return date changed, or to buy two singles?
To begin with your last question: two singles are almost always much more expensive than buying a return ticket, even when changing the date if possible. You are likely to pay 150% in that case in general, although on some routes you might get lucky.
I know that cheapest prices for tickets to New Zealand (or Australia) from Amsterdam are not as you might think in their winter only. You can find prices to be the same in summer, although Christmas to January might be a bit more expansive.
Last summer (I mean their summer) I paid around 1050 euro (about 750 pound) for a ticket to Oz including all taxes. The same price applied to New Zealand. It was with Cathay Pacific, which has good deals almost yearround, check their website.
Also Qantas, Malaysia and Singapore Airlines offer good deals, although less frequently was my experience.
A return ticket via America is mostly with Air New Zealand but it's generally more expensive.
Many airlines will let you change your date for free but charge you for any change to your route. I flew with Qantas to Australia and NZ and changed my dates on numerous occasions without charge.
The more in advance you book the better deal you will get. For my last few trips I have booked with Trailfinders as they got me the cheapest deal.
Also bear in mind that on a tourist visa you would not be able to enter NZ on a one way ticket anyway - you must have your onward travel booked. This is waived if you have a working holiday visa.