I certainly hope that Auckland never sprawls much beyond its existing boundaries. The only places that I consider vaguely appropriate for further greenfields development are Papakura to Pukekohe and Swanson to Kumeu. However, the problem is that would take up a lot of incredibly productive rural land, that Auckland desperately needs.
Regarding a big upper North Island metropolis, this certainly won't (and shouldn't) happen by each city growing to meet others. However, towns between Auckland, Hamilton and Tauranga should definitely be encouraged to grow (in sustainable ways). A 160 kph regular train service between the three cities - servicing places like Pokeno, Te Kauwhata, Huntly (dare I say it), Ngaruwahia, Morrinsville etc. would enable people who want a more spacious lifestyle to enjoy it in those smaller towns, while the bigger cities intensity.
There would be huge economic advantages of being able to catch an hourly train service to Hamilton and/or Tauranga which took say 1 hour to get to Hamilton and another 40 min to Tauranga. You'd be getting a faster city-centre to city-centre time than you manage on the plane I would imagine.