Hi Jeremy,
I lived in Auckland for a few years before moving over the Sydney last year. The area around Auckland has some great spots. My favorite is Muriwai Beach, especially the rock formations that lie between the north and south beaches at Muriwai. It's an easy 30 minutes drive from downtown Auckland, and you can park really close to where I'd set up my tripod for sunset.
Another great beach is Bethells Beach. This is a bit further way (40-50 minutes from downtown Auckland) but is wonderful. The drive to get there is part of the fun, as it's a winding road through the hills until you get to the Tasman; no mobile phone service out there. The beach is big, and even after parking it would take a good 20-25 minutes to walk to the rock formations at the south end of the beach. Only worth going out here if sunset is at or near low tide. Otherwise you can't get to the rock outcroppings unless you are willing to get very wet--and the waves area really too rough to risk it.
As for Rotorua, possible consider 2 nights but only one day there. Not much to see other than Wai-o-Tapu, which has the iconic geothermal pool that everybody photographs. Get there early and you'll have it to yourself for a bit. The path to get there is a loop; suggest entering the "exit" of the loop to get to the main pool faster.
Also, FYI, driving in NZ takes longer than one would think. Max speed limit is 100km/h, and frequently you have to drive slower. Can't remember exactly, but Auckland to Rotorua is probably 4 hours. Auckland to Bay of Island is closer to 5.
Cheers,
Jeff