A group of psychiatrists and social workers formerly employed at an Australian offshore immigration detention center publicly accused the government on Tuesday of failing to act over systemic child sexual abuse at the controversial facility.

More than two dozen people who worked at the camp in the Pacific island nation of Nauru said in an open letter published by the Australian Broadcasting Corp. that the government knew of a string of allegations as early as 2013 but did nothing.

Australia has been criticised at home and abroad for its tough immigration policies, including sending asylum seekers to camps in impoverished Papua New Guinea and Nauru, where they face long periods of detention.

"The government of Australia and the Department of Immigration and Border Protection have tolerated the physical and sexual assault of children, and the sexual harassment and assault of vulnerable women in the center for more than 17 months," the workers said.

A report by the Australian Human Rights Commission in February found nearly 300 instances of actual or threatened self-harm among roughly 800 children held in Australia's offshore camps between January 2013 and March 2014.

Immigration Minister Peter Dutton has repeatedly dismissed concerns over child safety as politically biased and argues that changes recommended by advocates would place children at greater risk by encouraging people to make the dangerous boat journey.

A spokeswoman for Dutton's office did not comment specifically about the issues raised in the open letter.

Australia received 16,000 asylum applications last year, just less than 0.5 per cent of the 3.6 million lodged worldwide, UN figures show. However, the issues has remained one of the most divisive in Australian politics over the past two decades.

Polls show the policies, which were central to conservative Prime Minister Tony Abbott's successful election campaign in 2013, remain popular with voters.

Under Abbott's tough line on people smuggling, none of the thousands of asylum seekers held in the two countries will ever be eligible for resettlement in Australia, even if they are found to be genuine refugees.

The former staff members detailed in the letter instances of abuse they said had been promptly reported to the government but received no action.

In one instance from November 2013, a boy was found to have been sexually assaulted by a camp guard but the Australian government chose not to remove him from the camp despite concerns for his safety. The boy was assaulted again, they said.