It is one of the major problems with living this far from everywhere else - flights are hideously expensive.
Also - fuel prices have shot up over the last three years - flights everywhere are more expensive than they were a few years ago.
Are the one way flights more expensive than the returns that you are looking at right now? (I don't know which flights you are looking at and how many hops you are taking around the world) I suspect that the higher prices are because you are trying to book at the last minute (how far ahead did you book your holiday?), so a lot of the economy fares have already been fully booked, and, if you are looking to fly into Queenstown itself, rather than stopping in Christchurch or Wellington and driving the rest of the way, you are aiming to go to a high tourist destination in the middle of the ski season, so all fares will be bumped up
I've had a quick look on AirNZ, and a return flight now (i.e. next week, returning mid August) London to Queenstown is £2,300+ per adult; a single is just under £2,000.
It may also be worth a quick call to the airline (whichever one you choose to go with) and ask whether they do any deals for emigrants; AirNZ did three years ago (it wasn't much off, but every penny counted), but we had to book a one way ticket only.
Another alternative, if you think you could cope with a longer journey and lots of dozing in airports, is to look at multi stages around the world; I know that you can find cheaper flights if you jump between airlines and across multiple countries; for us, this wasn't an option, as I don't travel very well, and I just needed the most direct flight (being air sick for 27 hours and paying more was more preferable to paying less, but being air sick for 40+!).