New Zealand could not stop a woman who travelled to Syria last year, apparently on a humanitarian mission, from coming into the country, Prime Minister John Key says.
The dual New Zealand and Australian citizen reportedly travelled to Syria last year for what were believed to be humanitarian purposes.
The Sydney Morning Herald reports the woman wants to return to Australia and is offering to help authorities with information about networks in the region.
The woman is the latest in a string of Australians who have travelled to troubled areas and now want to return home.
Political commentators say Australian Prime Minister Tony Abbott is expected to present new laws to Parliament that would withdraw citizenship from dual nationals who go abroad to fight for extremists.
Prime Minister John Key said he had no details on the woman other than media reports and she had not reached out to New Zealand.
Mr Key told Morning Report New Zealand authorities would need to look into the case to see what laws apply.
"She's been working theoretically for an NGO but if there are suspicions that something else has been going on then obviously, technically, the new foreign fighters legislation might fit.
"But we couldn't stop her coming back to New Zealand because to do that we would have [to have] provision and capability to be able to cancel her citizenship and we don't have that. We didn't get parliamentary support for that ... she would theoretically be able to come back I think but we'll need to look through the whole case."
Radio New Zealand understands the woman would be questioned by the Security Intelligence Service if she returned although it is unlikely any formal action could be taken.
Labour Party foreign affairs spokesperson David Shearer said if the woman had a connection with Islamic State she could be useful to the authorities.
Australia citizenship crackdown plan criticised
Founder of the Australian Arabic Council Joseph Wakim said an absolute one-way rule might not be the best option.
"I think it would be naive to have an absolute rule that says 'no way, we've warned you, it's one-way, that's it.'
"There could be scenarios whereby defectors are genuine, and, with many conditions applying, can actually help Australians better understand why young people are following this path of destruction, and why they get disillusioned."
Political reporter for the Guardian Australia Gabrielle Chan said critics of Mr Abbott's latest anti-terror law plans feared it the country was walking away from its responsibilities.
"They would rather have them sitting in jail here, in Australia, and possibly helping out with de-radicalisation programmes, than floating around the world, recruiting other Australians, or carrying out attacks on Australians."
Gabrielle Chan said there are very few details so far about the proposed new laws.