To put it in perspective, flying Sydney to Auckland is like flying Chicago to Vegas, plus the added issue of customs/immigration. It's over an hour's flight (or a full day's drive) from Sydney to Melbourne. It's a 5 hour flight from Papeete to Auckland. In 2 weeks, you could see the highlights of New Zealand, Sydney and possibly Melbourne, but add Tahiti and you're spreading yourselves pretty thin.
In mid-July, you'll have about 10 hours of daylight, from 7.30-5.30 in Auckland and about the same in Melbourne, and about 7.00-5.00 in Sydney. That means long evenings in places where most of the attractions are outdoors.
I checked some weather data. Based on the last 15 years, expect daily high temperatures in Auckland and Melbourne to be mostly in the 50's with a 60% chance of rain (or fog).. A really warm day would be low 60's, but it's just as likely to get days , Auckland is near the northern tip of the country. Farther south would probably be cooler along the coast and much colder in the mountains. Sydney is about the same, except temps would be around 10 degrees higher.