Australian Prime Minister Malcolm Turnbull has rejected Amnesty International's claims that Australia's detention of asylum-seekers on the Pacific island of Nauru amounts to torture as "absolutely false".

Australia sends asylum-seekers trying to reach Australia by boat to Nauru and Papua New Guinea's Manus Island, but the camps' conditions have been widely criticised by refugee advocates and medical professionals.

The asylum-seekers are blocked from being resettled in Australia even if found to be refugees.

Amnesty said in a report yesterday that asylum-seekers and refugees on Nauru were "driven to absolute despair" and were struggling with an "epidemic of self-harm", adding that their living conditions fit international law's definition of torture.

But Mr Turnbull criticised the allegations and said his country's tough policies stopped people dying at sea on dangerous boat journeys.

"I reject that claim totally. That is absolutely false," he told Australian Broadcasting Corporation radio of Amnesty's charge of systematic neglect and cruelty on Nauru.

"The Australian government's commitment [to offshore processing of asylum-seekers] is compassionate and it's strong.

"As far as Nauru is concerned ... there is a very substantial investment there, to improve the circumstances of the people that are there."

Amnesty's senior director for research Anna Neistat, who travelled to Nauru for the report, said her organisation did not "come to these conclusions [about torture] easily".

"It is the nature of the harm, the fact that it is systematic... [and] deliberate, in the sense that it is being done with a clear purpose to create a deterrence, to punish," she told reporters in Sydney.

Ms Neistat slammed the veil of secrecy around the centres, saying she had to travel to Nauru in a personal capacity after her official applications were rejected six times.

Australia last year brought in secrecy provisions making it a crime for anyone who works for the immigration department, including contractors, to disclose information they obtain on the job, which critics say silences whistleblowers.

A former case manager on Nauru, Sandra Bartlett, said she was terrified about the legal consequences of speaking out, but that "more people will die" if action was not taken to remove them from the island.

The furore came as the ABC broadcast an investigation last night depicting the plight of refugee children living on Nauru.

Nauru's government has rejected the ABC report as "denigrating, racist, false and pure political activism" and "an insult to the people of Nauru".

The Nauru facility holds just over 400 men, women and children. Just over 800 asylum-seeker men are held on Manus, with Australia in August agreeing to close the camp following a PNG Supreme Court ruling.