Counter-terrorism police preparing to prosecute "bumbling Jihadi" Mark Taylor upon his return to New Zealand could be derailed by outdated laws, a legal expert believes.

Detectives with the National Security Investigation Team have been building a case against Taylor for at least seven months in anticipation of Taylor returning from Syria, where he moved to in 2014 to join Islamic State.

Police have approached media to request any communications with Taylor be handed over to help bring a case against him.

The information would be "very important at a national security level should he come back and face prosecution in New Zealand".

Taylor is being held by Kurdish-led Syrian Defence Force in northern Syria, but his continuing captivity was thrown into doubt after Turkey invaded the area earlier this week.

Turkey has said that if its forces were to take control of Isis prisoners being held by the Kurds, it would seek to return them to their country of origin.

Assisting police with their inquiry is the long evidentiary trail Taylor has left on social media and in public statements during five years living inside an Isis-controlled Islamic Caliphate, and his subsequent imprisonment.

In 2015, he earned the nickname the "bumbling Jihadi" for accidentally giving away the coordinates of Islamic State fighters on Twitter.

University of Otago law professor Andrew Geddis said the Terrorism Suppression Act under which police would likely bring charges was "messy" and outdated, making it difficult for the Government to prosecute Taylor.

"It's messy primarily because the main statute of the act is a terrible bit of law," Geddis said. "The Government is reviewing it and for pretty good reason because applying it particularly to this bloke is difficult.

"Simply helping Isis with its ground war against the Syrian state may not rise to the sufficient level.

"The problem is there doesn't seem to be any evidence that he's committed a terrorist act. Standing around a fuel dump with an AK-47 isn't a terrorist act."

Geddis said police could look to charge Taylor for participating in a terrorist group, but under the legislation Geddis says prosecutors would have to prove Taylor was "enhancing the ability of the group to carry out a terrorist act".

"What exactly that means we don't know, because it's never been prosecuted in New Zealand."

Another potential charge would be attempting to recruit others to a terrorist group.

Taylor is believed to have set up a Linkedin account in late 2015 where he claimed to be teaching English to children in Isis-controlled Raqqa.

"Living in the heart of the Islamic State is a good experience and I encourage others to come and see for themselves," he wrote on the account.

Geddis said it was important to hold Taylor to account.

"If you don't prosecute him, it wouldn't send a good message that someone can play a relatively active role in a group as ugly as Isis was an walk away scot-free."

In July 2014, Taylor travelled to Syria, despite being monitored by New Zealand government agencies. He said then he would remain in Syria until he achieved martyrdom and posted pictures of himself to a Facebook account in traditional Muslim attire and carrying an AK-47.

In spite of his terrorist connections, Taylor had been an avid social media user. In the past, he has used his online accounts to announce he'd burned his New Zealand passport and to encourage others to wage jihad on Anzac Day.

From his former bolthole in Raqqa, Syria, Taylor sent photos and gave regular updates on his situation between 2014 and 2017.

As well as claiming to be an English teacher, Taylor posted pictures to social media showing him burning his passport, encouraging violence against Anzac Day parades, and taunting politicians.

After the Isis caliphate fell, he was imprisoned by Kurdish forces in northern Syria.

In interviews since being locked up Taylor has struck a more conciliatory tone, appealing to Prime Minister Jacinda Ardern that he be allowed back to New Zealand and suggesting he wants to set up a cannabis dispensary.

Ardern has said that the Government would not offer any assistance to bring Taylor home.

Former Defence Minister Wayne Mapp said it was in New Zealand's interests to bring Taylor back to face prosecution.

"It's taking responsibility to do our part in relation to known offenders of Isis.

"He went there to join the holy war, the jihad, and he knew he was doing that."

Mapp said New Zealand had a long tradition of extraditing criminals to face justice, and Taylor's case should be no different.

"They're declared under our legislation as a terrorist group. Someone who's joined a terrorist group should be prosecuted."

A police spokeswoman said they couldn't comment on Taylor's case, but said any citizen suspected of associating with a terrorist group overseas would be investigated under New Zealand law.

"The circumstances of these individuals are highly complex and any investigation or possible judicial proceedings would be considered on a case by case basis."

A spokeswoman for Justice Minister Andrew Little said the review of counter-terrorism laws was still ongoing.