A refugee who wrote an award-winning book while being held in detention has overstayed his visa in New Zealand, according to at least one official and the man's lawyer, in a move that could fuel diplomatic tensions with Australia.

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Behrouz Boochani travelled from Papua New Guinea in November on a temporary one-month visa to speak at a literary festival about his book, which details the six years he spent held against his will at Australia's notorious offshore detention camp on Papua New Guinea's Manus Island.

Ben Lomai, a lawyer in Papua New Guinea who has been acting for Boochani and hundreds of other refugees, told The Associated Press this week that Boochani had not returned to Papua New Guinea. A New Zealand government official, who was not authorised to speak publicly about the situation but was in a position to know, told the AP Boochani had remained in Christchurch after his visa expired.

Boochani has kept a low profile since his November 29 speaking engagement at the Word Christchurch festival and has not publicly stated his plans. Many people assume he will seek asylum in New Zealand.

Australian Home Affairs Minister Peter Dutton has vowed that Boochani will never be allowed to enter Australia, even if New Zealand ends up granting him asylum. That would create an unusual situation, because typically New Zealanders and Australians are free to live and work in both countries under a mutual agreement.

The situation also has raised questions about how Boochani managed to enter the country without Prime Minister Jacinda Ardern being told that he was coming.

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Since taking power two years ago, Ardern has several times repeated her country's longstanding offer to resettle 150 of Australia's offshore refugees. But Australia, which maintains a hard-line policy of never allowing boat refugees to set foot on its shores, has rejected the offer on the basis that it could open a pathway for the refugees to eventually settle in Australia.

A statement from Ardern's office to the AP appeared to add further confirmation that Boochani remains in New Zealand.

The statement said the prime minister doesn’t believe that Boochani remaining in the country will cause any problems for New Zealand’s close relationship with Australia. Her office referred further questions to immigration officials.

Greg Patchell, the head of Immigration New Zealand, said the agency couldn't comment on Boochani's visa status due to legal and privacy reasons.

Boochani could not be immediately reached for this story. When he was interviewed by the AP last month, he said he had no interest in returning to Papua New Guinea but wouldn't be drawn on whether he would seek asylum in New Zealand.

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“I’m really focused on my work here right now. I don’t want to think about those things, or ruin my concentration. I don’t want to politicize things,” Boochani said at the time. “I’m a free man. I want to focus on this festival.”

An ethnic Kurd and journalist, the 36-year-old Boochani fled from Iran, eventually making his way by boat to Australia’s Christmas Island. But his escape was not to freedom.

On Manus, Boochani helped shine a light on the plight of hundreds of asylum seekers by writing about his experiences on a smuggled phone and posting to social media.

He documented unsanitary conditions, hunger strikes and violence, as well as deaths caused by medical neglect and suicide. He said he felt a responsibility to film and write, to challenge the system and expose what was going on. It gave him some catharsis.

He eventually used his phone to write his book, sending snippets in Farsi to a translator over the messaging app WhatsApp. Called No Friend But the Mountains, the book this year won a prestigious Australian award, the Victorian Prize for Literature.

But Boochani couldn’t collect his award or the prize money of NZ$130,485 in person because he was still confined to Manus. He was later moved the capital, Port Moresby.

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Ardern said she wasn’t told by her immigration minister that Boochani was coming and would have liked a heads-up. Immigration Minister Iain Lees-Galloway, in turn, blamed immigration officials, saying they initially told him that Boochani had applied for a visa but then neglected to tell him they'd approved it.

“I was disappointed about this, and would have liked to have been able to provide a heads-up for the prime minister," Lees-Galloway said in a statement. “I have spoken with Immigration New Zealand about this and they have apologised.”

Complicating matters, some lawmakers from both New Zealand and Australia knew about the trip in advance. Boochani was greeted at Auckland Airport by the Green Party's Golriz Ghahraman and spent several days traveling with Australian Greens Senator Nick McKim.

McKim told the AP he considered himself friends with Boochani but wasn't going to speak publicly about the immigration issue.

Ghahraman said she wasn't involved with Boochani's immigration plight but said it had been a “real joy” getting to know him better and supporting him at events.

Boochani's trip was sponsored by Amnesty International New Zealand. Executive Director Meg de Ronde said in an email she was limited in what she could say.

“We were thrilled to sponsor Behrouz's visitor visa to the Christchurch Word event to discuss his amazing work, and any other matters are up to him,” she said.

Lomai, the Papua New Guinea lawyer, said Boochani's case was unusual because most refugees weren't allowed to travel freely within Papua New Guinea much less get a visa to travel abroad. But he said the Papua New Guinea government likely figured it would save money and prevent more bad publicity if it allowed Boochani to leave. Lomai said Boochani would be put back in detention if he ever returned.