A former US marine pilot, arrested under a veil of secrecy is being held under strict conditions inside a maximum-security prison in Australia, according to his lawyers.

Daniel Edmund Duggan, who has been fighting his extradition to the US on unknown charges, is due to be transported to Goulburn's Supermax prison while being classified as a "extreme high risk prisoner", reported Australian broadcaster ABC news.

His defence lawyer, Dennis Miralis said that Duggan was wrongly classified and made a request for the attorney-general to release him.

He also suggested that "foreign interference" by the US government might have led to the harsh treatment Duggan has experienced in Sydney.

Miralis said that the Australian has been denied access to writing material and medical treatment.

"He's presently not even able to access pens for the purposes of writing the nature of his complaint," Miralis told the court.

Duggan's lawyer calls his arrest — 'unprecedented'

Duggan was arrested in New South Wales on October 21, the same week that Britain and Australia issued warnings about China's attempts to recruit military pilots.

The British media had reported that over 30 pilots had accepted lucrative offers to train the Chinese military. China's foreign ministry denied any knowledge of the allegations made by two countries.

According to Duggan's company website, Topgun Australia, he has spent more than a decade flying in the US Marine Corps where he reached the rank of major and worked as a tactical flight instructor.

After leaving the US Marines, Duggan moved to Australia where he ran the 'Top Gun' adventure flight company from the Southern Island of Tasmania. In 2014, five years before his arrest, the 54 year-old had lived and worked in China as an aviation consultant.

Duggan's lawyer explained that currently, Duggan is an Australian citizen and had renounced his US citizenship.

Reuters news agency reported that when Duggan was in China, he shared his address with Chinese businessman Su Bin. In 2016, Bin wasjailed in the US for his involvement in a high-profile hacking case regarding the theft of US military aircraft designs.

Previously Duggan and Su Bin also worked for a South African flight school which is now under scrutiny by British authorities for training Chinese military pilots.

Duggan has been detained under Australia's Extradition Act, pending an official request from the US government. He has not been told his charges and details regarding his alleged offences remain sealed by the US government.

"This is unprecedented to have an Australian citizen placed on the most strict inmate restrictions, akin to people convicted of terrorist offences," said Miralis.

US government calls it a 'usual extradition process'

Earlier this month Miralis raised concerns about the  "dramatic and aggressive move" to transfer Duggan to a maximum-security prison. He launched a complaint regarding the Australian's treatment in court.

Miralis also said that he would file a complaint over the conduct of Australian intelligence officers during Duggan's arrest. He added that, until the complaint is resolved, Duggan's extradition should be put on hold.

"Mr Duggan is an Australian citizen. We ask the US not to interfere," said Miralis in early November.

Lawyer Trent Glover, appearing for the US government, said the Department of Justice would file an extradition request before December 20. "From the United States' point of view, this is nothing more than a usual extradition process," he told the court.

Duggan's case will be heard in court in late December.

ns/kb (AFP, Reuters)