New Zealand is considering detaining people with terrorist links on the grounds they may commit crimes.
Prime Minister John Key said on Sunday he was concerned there was a risk of New Zealanders travelling to Iraq or Syria to support the Islamic State group and then coming home.
It is estimated there are 3000 foreign fighters, including Australians, who have joined the group's cause.
New Zealand's cabinet is meeting on Monday where it will consider urgent changes to deal with the issue.
Unlike in Australia, it was not a criminal act in New Zealand to go offshore and fight for a terrorist group and passports could only be cancelled for 12 months, Mr Key told TVNZ's Q+A.
"At the moment we have very little rights if someone says they want to get up and go and fight for a terrorist group."
Last week, a dual New Zealand and Australian passport holder was detained in Australia under their law, he said.
Hypothetically, if that man fought for ISIS he would be far more likely to return to New Zealand where it is not illegal, Mr Key said.
"Then the question is what domestic threat does an individual like that pose."
Cabinet will consider what changes should occur rapidly.
"Potentially we would have greater powers and potentially even powers to look at arresting someone under the view that they would undertake what would then be deemed to be a criminal act.
"That's a very big step - I'm not saying we will take that."
Mr Key said it would be odd for New Zealand to do nothing in the fight against the "dangerous and growing" terrorist group.
He had been advised joining the fight would not change the risk of a terror attack in New Zealand in a material way.
"If the reason you don't do that is because you feel intimidated by them, then by definition they are starting to control your foreign policy."
Options ranged from humanitarian aid, diplomacy, military airlift capability, to ground troops, Mr Key said.
New Zealand is reassessing its terror risk threat level, which is kept secret.
"It's likely to rise," Mr Key said.
It would not be as high as Australia which recently raised its level from medium to high.
