A lawyer says a former Kiwi soldier who joined Islamic State and is detained in a Syrian prison is still New Zealand's responsibility.

Mark Taylor, also known as Mohammad Daniel and Abu Abdul Rahman, burnt his New Zealand passport after going to Syria to fight for Isis.

In 2014, Taylor, declared a Specially Designated Global Terrorist by the US, claimed he'd contacted the New Zealand Government to try to get a new passport.

At her post-Cabinet press conference, prime Minister Jacinda Ardern says Taylor can potentially return home, but he would face legal action here. 

Auckland University law professor Bill Hodge told Larry Williams that we would have to take him back if he wanted to return. 

"He's got New Zealand citizenship, there is only a narrow ability under the Citizenship Act to take that away."

Hodge said it would be "drawing a very long bow" to take his citizenship away. If he had gained citizenship in another country, it would be possible, and Hodges says the Government could argue he became a citizen of ISIS. 

"They could say 'they behaved like a state, you acted like a citizen of the state, you burnt your passport'." 

Ardern says if the ISIS supporter wants to come home, he would have to be issued emergency travel documentation.

However, gaining access to such documentation would prove difficult for Taylor.

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"New Zealand does not have diplomatic representation in Syria, and the ability of the Government to assist New Zealand citizens is severely limited."

The Prime Minister said she could not get into hypotheticals around if Taylor would be coming back to New Zealand.

Ardern said at the press conference that Taylor's actions created the potential for legal actions in New Zealand.

Hodge says that, should Taylor return, there is an "extra territorial" statute that allows New Zealand to punish someone for crimes they committed overseas.

"Under the Terrorism Suppression Act of 2002, participating in a terrorist group can get you 14 years back here." 

The case would require a lot of evidence, but Hodge says that he is not "the sharpest tool in the shed" and he keeps talking about his potential crimes. 

"The evidence may lie out of his own mouth", he says. 

Hodge says that the recordings and statements Taylor has made would be the only evidence they could use or need.