2 hours should be okay. However, if weather in Queenstown stops flights (less likely in January than this month but still happens) then you probably won't make the Qantas flight. Luckily there are later flights to Sydney, but you may have to wait a while if they are full (quite possible in January). If you could switch to CHC-SYD that would be safer (or AKL-SYD on NZ).