New Zealand women who have left the country to marry jihadist fighters in the Middle East have not had their passports cancelled and are free to return, the Government says.

New Zealand's Security Intelligence Service (SIS) director Rebecca Kitteridge revealed this week that a growing number of women were heading to Iraq and Syria.

She said it was not clear whether these "jihadi brides" had gone to the region to fight themselves or to support Isis (Islamic State) fighters.

The number of New Zealand women in this category was not known, but it was less than a dozen.

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Counter-terrorism law changes passed a year ago gave the Internal Affairs Minister greater powers to suspend or cancel the passports of suspected foreign fighters.

Internal Affairs Minister Peter Dunne said no woman's passport had been cancelled in any circumstances.

A very small number of men's passports had been cancelled, though he would not reveal the number.

"I'm not going to go into any particular details, but any passports that have been cancelled have been cancelled because people pose a threat to national security or who are going to engage in terrorist activities. Marriage doesn't usually come into that category."

Ms Kitteridge said that if any of the women returned from Iraq or Syria, the SIS would "maintain an interest in those people".

"Obviously we would be concerned with whatever they are doing in a war zone of that kind.

"There would be really significant concern about what they are being exposed to, the conditions that they are in, their ability to get away if they want to or how heavily radicalised or exposed to acts of barbarism they might be seeing."

She made the comments to the intelligence and security committee at Parliament, as part of the SIS' annual review.

Mr Dunne, speaking as United Future leader and not in his ministerial capacity, questioned the spy agencies' warnings about "jihadi brides".

In a blog published today, he said the number of New Zealand women heading to Iraq and Syria was very small, and New Zealand's domestic terror alert status had not changed.

"Therefore, why raise this spectre of alarm? One reason might be that, small as it is, the number has grown, and is consequently, in a time of increased international tension, worth noting. Possible, but unlikely."

He said Ms Kitteridge's comments appeared to be part of a "softening up process" for the outcome of an independent review of New Zealand's security agencies.

"After all, heightening the perception of threat would boost the case for increasing the powers of the security agencies," he said.

"This is a little too obvious and we should be careful not to become too taken in by it."The review was expected to be released early next year.