Queensland barrister Greg Egan and his junior counsel Terence Lambert, who are representing PNG fraud investigators, argue their travel ban is "a ploy" to further undermine the case against Mr O'Neill.

Australia's Department of Foreign Affairs and Trade has declined to intervene, saying the case is "a private legal matter" between the men and the PNG government, and Australia "must respect PNG legal processes".

A letter signed by PNG's Chief Migration Officer Mataio Rabura​ on September 8, seen by Fairfax Media, instructs international airlines not to carry the men on any flights due to land in PNG, or risk penalties.

Australian and PNG legal bodies have raised concerns over the travel bans, saying they could be viewed as an attempt to pervert the course of justice.

Last year, investigators served an arrest warrant on Mr O'Neill, after an investigation by police and anti-corruption agency Taskforce Sweep. Mr O'Neill responded by sacking the country's Attorney-General and others.

As efforts to have the arrest warrant executed continued, the Australian lawyers were told on September 16 they had been banned from entering PNG at the order of Mr Rabura.

Mr Egan, who has been conducting law in PNG since 1988, and Mr Lambert referred the matter to the Department of Foreign Affairs and Trade in the hope it could be resolved at a diplomatic level.

But with no progress on that front, they last week lodged papers with the PNG National Court of Justice for a judicial review of the case.

"The imposition of this ban on lawyers representing the legitimate interests of people in PNG, to my memory, is unprecedented," Mr Egan said.

"We had hoped that the matter could have been resolved diplomatically before it progressed to the stage where we were required to institute proceedings. The people of PNG need support on many fronts ... The last thing they need is a breakdown of the rule of law."

Australian Bar Association vice-president Patrick O'Sullivan​ said: "Interfering with the impartial administration of justice will only serve to jeopardise the rule of law in Papua New Guinea [and] I urge the PNG government to see what is at stake."

PNG's Law Society president Peter Kuman said in a statement the lawyers held valid practising certificates allowing them to operate in the country.

"All lawyers in this country must be able to practise law in this country without fear or favour."

A spokesman for Mr O'Neill said that as the matter is subject to legal proceedings, he could not comment on it.

Efforts to reach Mr Rabura were unsuccessful, but he told the ABC's Pacific Beat program in a statement that the men had been entering the country on the wrong visa.

"These two individuals have been entering the country on business visas but have in fact been engaged in employment."

The Australians dispute this.