Manus Island detainees will receive $70 million in the settlement of a class action against the Australian government.

Lawyers for the detainees have reached a conditional settlement with the Australian government and the operators of the Manus Island Regional Processing Centre.

Slater and Gordon principal lawyer Andrew Baker said the case had been settled for $70 million plus costs.

"After three years investigation and legal action, the Commonwealth and its subcontractors, G4S and broad-spectrum, have agreed to a settlement of $70 million plus costs," Mr Baker said during a press conference this morning.

"The Senate said this was the largest settlement in a human rights class-action in Australian legal history and we think this is true."

The class action was seeking damages on behalf of 1905 group members who, legal firm Slater and Gordon says, cover the majority of people detained on Manus Island since 2012.

The detainees wanted compensation from the Australian government and the detention centre managers for alleged physical and psychological injuries they argue they suffered as a result of the conditions in which they were held.

The class action also sought damages for false imprisonment after the PNG Supreme Court ruled the detention of asylum seekers on Manus Island was unconstitutional.

The Australian government and the companies that have managed the Manus Island Regional Processing Centre, G4S Australia and Broadspectrum (formerly Transfield Services), had denied the claims.

The settlement could potentially lead to the withdrawal of a separate legal action in Papua New Guinea alleging false imprisonment against the PNG government, which lawyers estimated could be worth at least $150 million for about 1000 detainees.

Lawyers Greg Toop and Ben Lomai have estimated the men in the PNG case could each receive more than 400,000 PNG kina ($A150,000-$A175,000).

The settlement details in the Australian class action will be confidential and subject to court approval.