Australia would “flagrantly” violate the refugee convention if 153 Tamil asylum seekers on board a boat headed to Australia were sent to Sri Lanka without being allowed to lodge a claim for protection, the Refugee Council of Australia has warned.

The Christmas Island shire president, Gordon Thomson, told Guardian Australia on Tuesday there were unconfirmed reports on the island that those on board the boat had been handed over to the Sri Lankan navy.

Thomson said it was clear that those on board had not yet been brought ashore and there was no sign of the boat.

The Sri Lankan high commissioner in Canberra, Admiral Thisara Samarasinghe, told Guardian Australia on Tuesday evening he had not been informed by the Australian government of the boat's existence and he was in no position to comment without knowing the facts, but was unaware of any reports of the Sri Lankan navy being engaged.

He said the Sri Lankan government continued to support Australia's border policy "because we don't want people dying at sea and we want to stop people-smuggling".

Australia and Sri Lanka enjoy close diplomatic relations, which allow for the the involuntarily return of any asylum seeker from Sri Lanka with a failed claim.

The office of the immigration minister, Scott Morrison, declined to comment on whether a handover had occurred.

Morrison has given no detail on the boat’s whereabouts and refused even to confirm its existence since refugee advocates revealed last Friday it was heading to Australia from Tamil Nadu in southern India.

Sri Lankan asylum seekers are subject to the "enhanced screening process" in Australia, which has been condemned by the UNHCR as an “unfair and unreliable” process for determining refugee claims as it involves short interviews, often without the presence of a lawyer.

In October, Morrison said he was “completely comfortable about the process”, adding: “If you're coming here to try something on to get access to Australia from Sri Lanka, you'll go straight back.”

The Refugee Council’s chief executive, Paul Power, said Australia would potentially be sending asylum seekers back to “persecution or worse” if the Tamils on board were returned to Sri Lanka.

“As a Refugee Convention signatory, Australia has a clear obligation not to send asylum seekers back to danger without giving them a chance to put their case for refugee protection,” he said in a statement. “Forcing people seeking refugee protection back to their country of origin without properly investigating their claims is the most flagrant of all breaches of the Refugee Convention and international law.

“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.”

Ian Rintoul of the Refugee Action Coalition, who had been in regular contact with those on the boat until communication was lost on Saturday, said there were 37 children on board, some as young as a year old.

It is unclear what citizenship these children would have if born in India, the boat’s point of departure. India is not a signatory to the refugee convention.

Greens senator Sarah Hanson-Young said the unconfirmed reports that those on board had been handed to the Sri Lankan navy were “extremely concerning”.

“The Australian government has a responsibility to assess these people’s claims for protection, not hand them over to the very authorities they are fleeing,” Hanson-Young said.

“The minister must put an end to the rumours and tell the Australian people what is really going on.

“There are children on board these boats, we must ensure their safety and offer them protection.