You'll probably find a return to be cheaper than a one-way. AKL-ZQN return, leaving the 20th, returning the 30th is NZ$499 (US$200).
AKL-CHC (Out May 20th back May 31st) is showing at NZ$359 (US$145). I got these prices from
www.airnz.co.nz
Also, Air NZ have just started a domestic service with their budget airline Freedom Air. They're quoting fares as low as NZ$178 (US$72) AKL-CHC return (They don't do domestic one-way fares). They're on
www.freedomair.com
You won't find any NZ consolidators able to do a better price, I'm afraid. AKL-ZQN is a 1hr 50min flight, AKL-CHC is 1hr 30min.