That sounds real crazy like if the Canadians could come fly domestic in USA and Vice versa WTF...............
Same rule that allows Jetstar to fly Domestically within New Zealand with its VH-*** registered aircraft.
NZ/AU is operated as an single aviation market, any NZ/AU carrier can operate in either country.
Just like Virgin Australia, 10x of the 738s are ZK- registered, from time to time they will end up on AU domestici when needed.