New Zealand has a "broad sense" of the number of Kiwis fighting with Islamic State and has monitoring in place in case they tried to return home, Prime Minister John Key says.

Key met with Australian Foreign Minister Julie Bishop on the weekend during her brief visit to Auckland.

Afterwards, Bishop told the Herald that about 110 Australians are fighting with isis in Iraq and Syria, and concern is turning to what to do with them if they survive and return home.

Isis was under increasing pressure, particularly because of the Mosul offensive, and there was a "high risk" that more foreign fighters would try and return to Australia, Bishop said.


Asked about those comments today, Key said intelligence agencies had a "broad sense" of the number of foreign fighters with links to New Zealand.

"It's obviously considerably smaller than Australia's issue," Key said after his travel to India was delayed by plane troubles during a refuelling stop in Townsville, Australia.

"Whether you will ultimately see a return of foreign fighters, we don't know. That is always a risk, because they are legally entitled to return to New Zealand."

Last December a law change was made that aimed at stopping would-be foreign fighters from leaving New Zealand to join Isis in Iraq or from carrying out terrorist acts in New Zealand.

The Countering Terrorist Fighters Legislation Bill gave the SIS greater powers of surveillance and to give the Minister of Internal Affairs greater powers to suspend and cancel passports.

"Realistically all we can do is be aware of the people...monitor them and see where they return to," Key said today.

"Some of them are dual passport holders, so they may attempt to go back to Australia if that's not possible, then we may end up seeing them in New Zealand. But of course New Zealand and Australia share information on these fronts."

In 2014 John Key said five New Zealanders were among the foreign fighters and 30 to 40 New Zealanders were on a security watchlist as potential foreign fighters, with another 30 to 40 needing to be investigated.

New Zealand and Australia - with 100 and 300 troops respectively - have trained 12,000 Iraqi troops at Camp Taji near Baghdad, many of whom are taking part in the offensive to reclaim Mosul, which ISIS seized two years ago.