New Zealand and Italian scientists have worked together to show that killer whales swim 4900km from Antarctica to Northland - a discovery that has turned previous thinking on its head.

There are several different kinds of killer whales, also called orca, and it was thought that "type C", numbering an estimated 300-3000, lived out their lives in the Ross Sea.

A separate, well-documented type inhabits New Zealand waters.

Now University of Canterbury scientists and Antarctic researchers from Italy have uncovered the whales' commuting secrets.

Gateway Antarctica scientist Dr Regina Eisert led a team at Scott Base during the 2014-2015 Antarctic summer that studied killer whales as part of a larger research programme on the Ross Sea ecosystem involving NIWA, Landcare Research and Lincoln University.

"We wanted to determine whether a decline in the toothfish fishery in the Ross Sea poses a risk to Type-C killer whales, including finding out how many there are in the Ross Sea and where they feed," she said.

At the same time, about 360km north of Scott Base, Italian whale experts Dr Giancarlo Lauriano and Dr Simone Panigada deployed satellite transmitters on killer whales in Terra Nova Bay to determine the whales' movements.

Eisert said the two teams hit "research gold" when their results independently verified last month that type-C killer whales were commuting between Scott Base and the waters off Northland.

Canterbury University student Ekaterina Ovsyanikova, whose work is supervised by Eisert and killer whale expert Dr Ingrid Visser from the Orca Research Trust, discovered that the same female type-C killer whale had been photographed repeatedly in New Zealand and in McMurdo Sound, Antarctica.

"This suggested that the killer whale had been commuting between Scott Base and Northland," Eisert said.

"Satellite data received by Dr Lauriano and Dr Panigada showed that the whales swam due north towards New Zealand, confirming beyond a doubt the findings suggested by photo-identification of Ovsyanikova and Visser."

This crucially changed the scientists' understanding of the ecology of these key top predators and the potential threats they might face.

"The whales' long commute would also suggest that there is much greater ecological connectivity between Antarctica and New Zealand than previously thought," Eisert said.

There are still many questions.

The satellite tags only transmit for about a month. The whales took 22 days to reach Northland.

One group turned back towards Antarctica but the tags stopped sending after a few hundred kilometres. A second group swam east and then west again, in a large circle, before contact was also lost.

Where they went next is something that Eisert is keen to find out.

"We have no idea. Once the cat's out of the bag that these Antarctic killer whales do not stay in the Ross Sea all the time, all bets are off. It's very exciting."

Among other things the scientists want to learn where the whales have their calves. Very small calves have been seen in both McMurdo Sound, Antarctica, and off Northland. Because they can travel almost effortlessly, '"drafted" by their mothers, it's possible that they could have been born in place.

"There is a possibility that they go to the northern waters to give birth and then draft the calf back to Antarctica - but that's speculation right now," Eisert said.

"We've seen tiny, tiny babies down there - so small that they looked like a killer whale's pet dolphin."

She said the Antarctic ecosystem project was long-term and she was hoping that next summer's funding would come through in August.

"What I really need is for someone to give me $2 million so I can charter a boat, go to the Ross Sea and count the killer whales."