One thousand-plus asylum seekers being held by Australia in offshore detention centres are suing Canberra, alleging they have suffered torture and human rights abuses.
Migrants being held on the islands of Manus -- in Papua New Guinea -- and Nauru are to launch two class action suits, the latest civil bid to end their detention.
"The group members allege that they have been subjected to torture, crimes against humanity and the intentional infliction of harm by the Australian Government," said George Newhouse, a lawyer with legal campaign group the National Justice Project.
The roughly 1,200 individuals will be represented by top lawyer Julian Burnside, the project said.
Australia's right-leaning government has repeatedly defended its policy of detaining migrants at offshore facilities.
Prime Minister Scott Morrison argues allowing the would-be refugees to reach Australia would encourage others to make the trip.
But the policy is unpopular among the Australian public and has been pilloried by rights groups and doctors as inhumane.
Conditions in the camps are often difficult.
The United Nations and a doctors group recently warned of alarming levels of depression on Nauru, with many suicide attempts and children living in despair.
Those detained have already launched several legal challenges in a bid to close the camps.
Their lawyers will now argue they have suffered arbitrary imprisonment and denied proper medical treatment -- constituting a crime against humanity.
The government has vowed to move children off Nauru but has opposed legislation allowing transfers to take place.