I would expect permission is required from CAA of NZ prior to going there and flying around with an Aussie licence and A/C
No need to contact CAA.
Hmm definately don't want to pay GST on the plane (is that even possible?)
No need to pay GST, not aware of there ever being an issue in the past. Arrange Customs at KeriKeri, they come out to the airport from the port at Opua (near Paihia). They're very helpful but you will need to prearrange for them to meet you plus there will be a small to moderate fee. Still better than going to Auckland.
In terms of the fuel, I hold a BP carnet card and its my impression that this is widely available in NZ in terms of fuel bowsers. I have heard that the carnet cards don't work with the pumps though
Your best option is to get a BP swipe card before you leave Oz. Most places that have fuel have BP you just rock up anytime of the day swipe your card and go.
Are you aware if Australian ASIC cards would suffice to go airside over there?
We're not as anal over here as you guys are in OZ about this. Might be useful as they look similar to our ID cards, but probably not necessary, have your licence and or a photo ID like a passport will probably suffice.