Two bad-tempered Australians caused anxious moments for the crew of a Royal New Zealand Air Force Hercules.

Saltwater crocodiles Scar, from the Northern Territory, and Goldie, from Cairns in Queensland, were airborne on their way to a new life at Manukau's Butterfly Creek Zoo when they broke their head restraints, zoo general manager John Dowsett said.

"It was a bit fraught ... Two of them broke their head-ropes and began thrashing around. The plane was shaking. The pilot was turning and raising his eyebrows.

"We gave them a muscle relaxant to calm them down. But they're big animals - when they do move, there's an impact. There's a lot of power there."

The crocodiles eventually calmed down, and the flight landed at Whenuapai Airbase about midnight on Tuesday.

The Air Force Hercules was used to fly the crocs as it was the only available plane with enough room and it was returning to New Zealand from Asia, via Darwin.

Butterfly Creek, which is at Auckland Airport, was not charged for the flight, but Mr Dowsett hopes to host Air Force staff for a meal to thank them for their help.

Scar and Goldie will spend two weeks in quarantine at Butterfly Creek before beginning new roles as the stars of a saltwater crocodile exhibit.

And keepers will have to be careful to keep the pair apart, because the creatures feel the same about each other as they do about head restraints.

"We have two crocodile exhibits side by side, but we can't let them be together because they'll kill each other," Mr Dowsett said.

"You go out in the wild and most of the bigger crocodiles have legs missing or ripped eyes - they're very aggressive."

Scar and Goldie were selected from hundreds of crocodiles because of their good health and relatively stable personalities.

"They can't be pulled straight out of the wild," Mr Dowsett said.

"They've got to be confident without being overly aggressive. Otherwise they'll spend the whole day charging the fence. Not all the crocs are the same; some are absolutely crazy."

Transporting the crated crocodiles from Whenuapai to Butterfly Creek took more than five hours, and eight carers travelled with them throughout the move.

"Getting them out of the boxes can be a bit hairy," Mr Dowsett said.

"We pull them out with a top jaw-rope, jump on them while we undo the tape, and get off at the same time. You just hope they don't spin around."

The crocodiles are expected to be ready for public viewing in three to four weeks, once they are used to their new home and feeding patterns.

Butterfly Creek spokeswoman Jackie Sanders said the pair were the largest and most dangerous predators in New Zealand.

* Average lifespan 80 years
* Largest reptile
* Can go months without food

* Caught in Northern Territory; 40 years old.
* 4.8m long; weighs about 420kg.

* Raised on a farm in Cairns; 40 years old.
* 4.5m long; weighs about 450kg.