To put it bluntly NZ weather is variable all the time. We have to put up with your leftovers drifting across the Tasman.
Feburary - March is most stable but I'd suggest the South Island around Lake Tekapo as your best location. It's the only Gold Certified Dark Sky site in the world I think.
Google Mt John Observatory and you'll find out plenty. Only problem is it's miles from anywhere and accommodation will be expensive as you'll be in high season for holiday goers.
A few handy links for you.
http://www.tekapotourism.co.nz/info/mt_john.html
http://www.mtcooknz.com/mackenzie/pr...=earth-and-sky