"What it does mean is that, for the safety and security of New Zealanders and what may occur if they go into that environment and then return, that's of interest to us and we deal with that appropriately."
He did not believe a tougher law to deal with those returning was needed. Australia's laws include provisions to imprison those who enter a foreign state for hostile activities, or who plan to do so by training or stockpiling weapons. Mr Key said Australia faced a different situation because more people were involved.
"We believe we have enough powers to deal with the situation. Australia's numbers are much greater and they've taken a different perspective on it."
Australia's Federal Police have said up to 100 Australians were understood to be fighting in Syria, with about six killed "while fighting with al-Qaeda-linked terrorist groups".
Mr Key refused to say whether New Zealanders in Syria were linked to al-Qaeda groups, but said some were dual citizens with Australia.
He said it showed the importance of intelligence agencies' work.
"From time to time, we need to track the activities of New Zealanders, we need to be sure of their whereabouts and we certainly need to be clear that if they return to New Zealand, whether they pose a threat to other New Zealanders if they have become radicalised."
He said New Zealand sometimes shared its intelligence because of the groups the people were involved with.
Mr Key refused to provide details on those involved, but said a small number were there. He said that since 2005 the law had allowed for the Minister of Internal Affairs to cancel a passport in some circumstances.
He said that was a big step and fewer than 10 people had had their passports cancelled since 2005, including some who were believed to be heading to Syria to fight. By comparison, Australia has cancelled about 20 in the past year.
"They obviously don't put their hand up and say they're going to be freedom fighters in Syria when they leave. They present a different set of reasons why they might be leaving the country. We have the capacity to cancel a passport if we believe somebody is going into a war zone, for instance, to fight in a way we don't think is a sensible step for them to take."
He said it was also possible to cancel a passport on national security grounds after somebody had left the country. As a result, they would have to stay where they were or be deported back to New Zealand.
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