Wellington (AFP) - New Zealand Prime Minister John Key warned Wednesday that Australia risks straining the trans-Tasman "special relationship" by deporting Kiwi-born convicts under tough new immigration rules.

Hundreds of New Zealanders are being held in migration centres awaiting deportation under new rules that say foreign nationals should be sent home if they have served a jail term of 12 months or more.

With some 650,000 New Zealanders living in Australia, there are concerns the numbers could rise steeply and that many people are being unfairly targeted.

The issue has been brought into focus by the suicide of Junior Togatuki -- a 23-year old who had served time for armed robbery and assault -- in an Australian prison this month.

Togatuki, who had mental health issues, moved to Australia when he was four and was awaiting deportation after completing a seven-year jail term when he killed himself.

Key said he was concerned about New Zealanders being sent to detention centres, including remote Christmas Island in the Indian Ocean, and had raised the issue with Australia Foreign Minister Julie Bishop.

"I was pretty blunt and I said there's a special relationship between New Zealand and Australia and you challenge that relationship to a degree when you see New Zealanders being treated in this way," he told Radio New Zealand.

The reason so many New Zealanders live in Australia is that both countries offer each other automatic residency rights, with many Kiwis making the trip to take advantage of Australia's strong economy.

However, they remain New Zealand citizens even though Key said some of those being deported had no ties to the land of their birth.

"They've often spent their entire life in Australia and went over there when they were very, very young," he said.

"It's like the Australians are saying 'we're going to pick and choose, we'll keep the ones we like but send back the ones we don't like'.

"Well... you have to take the rough with the smooth."

The New Zealand Herald said in an editorial that it was Australia's right to pursue a draconian deportation policy but it made no sense applying it to people who had served sentences for relatively minor offences.

"Placing someone in that situation simply because they have been convicted of, say, shoplifting, smacks of overkill," it said.

From 1788-1823 New South Wales was a British penal colony. The last of some 162,000 convicts were transported to Australia in 1868.