Nauru's justice minister could face charges in Australia's highest court over his refusal to grant a business visa to an Australian lawyer.

Human Rights lawyer Jay Williams represents a group of Nauruan opposition MPs who were arrested following an anti-government protest on the island.

Two of the men, former president Sprent Dabwido and Squire Jeremiah, were released today after spending four weeks in custody.

They were charged with unlawful assembly, riot and disturbing the legislature after protesting outside Nauru's parliament last month.

They face a maximum seven years in jail if convicted and have engaged the Sydney lawyer to represent them.

Last week, Nauru's chief justice Joni Madraiwiwi ruled that Mr Williams could not appear for the men due to a "conflict of interest" arising from his role in ongoing cases, including one involving 1,000 asylum seekers in the island's regional processing centre.

"Mr Williams is the subject of proceedings which directly relate to this matter and the court has yet to rule on the issue surrounding his position in other proceedings," Justice Madraiwiwi said.

Under the island's constitution, Nauruans are entitled to the counsel of their choice.

The application for a judicial review, obtained by the ABC, claims the refusal to grant Mr Williams a visa constitutes an abuse of power.

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If the chief justice allows the review to proceed, Nauru's justice minister David Adeang would potentially face charges of abuse of process and unlawful interference in the island's judiciary.

If the review is refused, the opposition MPs can take their case to Australia's High Court.

The June 16 protest was sparked following allegations aired on the ABC's 7.30 program that Nauru's justice minister colluded with an Australian phosphate dealer and solicited bribes for himself and other politicians, including Nauru's president Baron Waqa.

Hundreds of Nauruans protested outside the island's parliament, calling for the president and justice minister to resign.

Another opposition MP, Mathew Batsiua, was arrested after clashing with police.

Sorry, this video has expired Protests break out in front of Nauru parliament

He too has engaged Mr Williams as his lawyer.

Some opposition MPs who did not take part in the protest were also targeted.

Roland Kun and Kieren Keke had their passports confiscated by Nauruan authorities.

The New Zealand government has voiced concerns about the breakdown of law and order in Nauru.

Last week, Australia's Foreign Minister Julie Bishop said she had sought assurance from president Baron Waqa that Nauru's government would respect the rule of law.

"We will continue to review this situation very closely," she said.

Nauru's Supreme Court will decide on Monday whether Mr Williams will be allowed to appear on behalf of the MPs and eight others charged in relation to the protest.