The last asylum-seeker children in Australian mainland detention have been freed, the government said on Sunday, although dozens of others are still being held on the remote Pacific island of Nauru, AFP reports.

Under Canberra's harsh immigration policy, asylum-seekers who try to reach Australia by boat are turned back or sent to Pacific camps in Nauru and Papua New Guinea where they are held indefinitely while their refugee applications are processed.

They are blocked from resettling in Australia even if found to be refugees.

Canberra has been under pressure from rights groups to release children from the centers, with doctors and whistle-blowers saying the detention of asylum-seekers has left some struggling with mental health problems.