Agents are trained to be very carefull; it can cost the Airline mucho moolah if an inbound PAX gets rejected on arrival immigration ... it's better to be on the safe side.
Yes, for the first poster, airlines certainly wouldn't want to fall fowl of US immigration. But for NZ??? Not going to cost that much moolah to get someone back to AUS from NZ if they were rejected, although chances are that most Australian citizens will sail through (And I do realise that NZ can reject Australians with a criminal record IIRC.. but thats irrespective of return ticket or not)