Australia is refusing to comment on the fate of more than 200 Tamil asylum seekers reportedly intercepted at sea to the north-west of the country.
It is believed two boats carrying the asylum seekers were stopped by Australian authorities in the Indian Ocean and that some passengers were handed over to the Sri Lankan navy.
Refugee campaigners say it is a violation of international law.
They say at least 11 of those on board had been tortured in Sri Lanka.
Australia has been taking a tough approach on asylum seekers who try to reach the country by perilous sea journeys.
Hundreds of would-be migrants have died trying to make their way to Australia by boat in recent years.
The government has made no comment for what it says are "operational reasons" and will neither confirm nor deny the existence of the two boats carrying Tamils from Sri Lanka, says the BBC's Phil Mercer in Sydney.
But Australian media say that some have already had their refugee claims rejected and have been transferred to the Sri Lankan navy, he adds.
Forcing asylum seekers back to their country of origin without properly investigating their claims is a flagrant breach of the Refugee Convention and international law, the Refugee Council of Australia said.
Chief executive Paul Power said: "For asylum seekers, this is a matter of life and death, particularly in Sri Lanka which has a long history of political violence on a scale unimaginable to Australians."
Ministers say, however, that Australia is upholding its international obligations.
Tamil Refugee Council spokesman Aran Mylvaganam said he had spoken to relatives of some of those on the boats and that at least 11 had been arrested by Sri Lanka's intelligence forces "and had been tortured".
The UN refugee council earlier this week expressed "profound concern" about Australia's handling of the asylum seekers.
It has been six months since a vessel carrying refugees reached Australia after the military was called in to turn boats around.
Under current Australian policy, most asylum seekers who try to make their way to Australia by boat are sent to detention camps in Papua New Guinea or Nauru. If found to be refugees, they will be resettled there, not in Australia.
Australia says its asylum policy - which is also widely believed to involve towing back boats to Indonesian waters - is aimed at saving lives.
Our correspondent says stopping refugee boats from reaching the country was one of the main campaign pledges of Prime Minister Tony Abbott.
Sri Lanka has been under heavy international pressure over alleged human rights violations during the final phase of the war against Tamil separatists which ended in 2009.
Rights groups say Tamils still face violence at the hands of the military.