It would be a lot cheaper to get an AC return from AKL to YYZ with a stopover in YVR. And then once in North America, get a North American Air Pass.

A return on AC AKL to YYZ starts from NZ$2445. Will probably be better to do an open jaw returning from YVR instead for YYZ... unless you want to clock up the mileage and go back to YYZ from SEA.