The law could be changed to make it harder for people on New Zealand passports to return from countries such as Iraq and Syria if they have been fighting for militant groups such as the Islamic State, Prime Minister John Key says.
At a meeting last week chaired by American President Barack Obama, the United Nations Security Council called for all countries to make it a serious criminal offence for their citizens to fight abroad with militant groups.
Mr Key said New Zealand would "potentially" consider a law change.
"I guess as best as I can share the information I have with you, I think what I would say is that if I was to spell out to New Zealanders the exact number of people looking to leave and be foreign fighters, it would larger, I think, than New Zealanders would expect that number to be."
The Security Intelligence Service was dealing with legislation which has been on the books for decades, he said.
"I don't know necessarily all the areas where the officials might believe that we should take a slightly different and, some would argue, tougher response.
"But what I do know is it's an area of enough concern for us to ask the officials to provide us (with) that advice."
Mr Key said work on reforms would need to done in tandem with the Opposition and the Intelligence Security Committee.
He acknowledged New Zealand had an easier regime than Australia but said he would not describe it has a haven for those wanting to return home.
The United States has no specific law preventing individuals from joining such groups but has anti-terrorism laws it has used to prosecute those associated with such groups.
The Australian government is proposing to declare some parts of the world no-go zones, making it an offence to enter any area where it considers a listed terrorist organisation is engaged in hostile activity.