Australia is returning almost 50 Vietnamese asylum-seekers using a warship currently off the Asian nation's coast after intercepting them at sea, a report said on Friday, as Canberra enforces a tough immigration policy.

The asylum-seekers were found by customs and navy vessels north of Australia earlier this month before being transferred to amphibious landing ship HMAS Choules, The West Australian newspaper said.

The daily estimated that the cost of returning the asylum-seekers could reach Aus$1.4 million (US$1.1 million), adding that it was not known if they had already been transferred to local authorities.

A spokeswoman for the Immigration Minister Peter Dutton said she was aware of the report but told AFP "we don't comment on operational matters". A spokesman for the Defence Minister Kevin Andrews referred all questions to the immigration department.

The Australian government has refused to disclose details of its military-led operation to turn back vessels carrying asylum-seekers trying to enter the country.

But it said in January that "15 returns of various forms", including boats turned back to Indonesia and Sri Lanka, instances where asylum-seekers were taken back by foreign countries, and rescues at sea, had taken place since the start of the policy in September 2013.

Turn-back operations last year angered Indonesia, with tensions between the two countries growing after the Australian navy admitted entering the nation's territorial waters.

Since July 2013, Australia has sent asylum-seekers arriving on boats to detention centres on Papua New Guinea's Manus Island and Nauru.

They are denied resettlement in Australia even if they are found to be genuine refugees.

The newspaper report came several months after the High Court ruled that Canberra's detention of 157 Tamil asylum-seekers from Sri Lanka at sea for weeks in June last year was legal.

Their case centred around whether Australia had the power to remove asylum-seekers from its contiguous zone, just outside territorial waters, and send them to other countries. The asylum-seekers were eventually taken to Nauru.

Canberra has said the policy is necessary to stop asylum-seekers entering Australia by boat. They had previously been arriving almost daily in often unsafe wooden fishing vessels, with hundreds drowning en route.