There is no reason at all why you should book standard awards.
There is also no reason why you should even attempt to book this via United or United.com
You will be best off booking on continental.com, as routings are more flexible.
You will need to find space on LAX/SFO/YVR-AKL-PPT/NAN on Air New Zealand, but coach should be so hard to find. Try out continental.com and see whats available, and how far in advance.
I've had no issue finding Air NZ space last summer YVR-AKL-NAN/NOU-AKL-LAX, and again for summer 2012 via AKL to RAR.
I think Tahiti will be harder to find, because there are less flights from Auckland than there are to Fiji.