Even as a non-diver I would still choose Cairns / Great Barrier Reef over Auckland. Unless you are after an additional immigration stamp on your passport, its a very simple choice.

No offense to Aucklanders (if there is such a term), but I found Auckland to be a very dull and scaled-down version of Sydney. If I have to choose a major city/town in NZ to visit, I would pick Queenstown (or anywhere in the south island) over Auckland.