The UN refugee agency warned Australia on Friday that it could be breaking international law, amid reports it rejected boats carrying asylum-seekers.
Adrian Edwards, spokesman for the UN high commissioner for refugees, said the agency wanted an explanation after reports that the Australian navy forced boats back, and plans to buy more vessels to bolster such operations.
"UNHCR would be concerned by any policy or practice that involved pushing asylum-seeker boats back at sea without a proper consideration of individual needs for protection," Edwards told reporters, saying it was still seeking details from the government.
"Any such approach would raise significant issues and potentially place Australia in breach of its obligations under the 1951 Refugee Convention and other international law obligations," he added.
Australia's conservative Prime Minister Tony Abbott, who won power in September after campaigning to "stop the boats", has introduced the military-led Operation Sovereign Borders to crack down on asylum seekers coming by boat.
Indonesia says Australia has turned back at least one boat carrying asylum seekers to its shores, although The Australian newspaper said as many as five have been secretly returned.
"As past experience has shown, such practices are operationally difficult and potentially dangerous for all concerned," Edwards said.
On Friday, Abbott likened the fight against people smugglers to war. The government has declined to reveal details of the boat incidents.
It has also refused to confirm or deny that it is planning to buy 16 hard-hulled lifeboats to ferry asylum seekers to Indonesia.
Australia's tough policies have irked Jakarta, which has warned that turning boats back could breach Indonesian sovereignty, feeding into existing diplomatic spats.
Over the past decade, UNHCR has repeatedly crossed swords with Australian governments of all political stripes, including Abbott's Labor Party predecessors who also backed tough measures.
Australia argues that is on the front line of a wave of illegal immigration and needs to step up the fight against people smugglers who try to bring people in, often on unseaworthy wooden vessels heading from Indonesia.
It insists that tough measures are needed to discourage illegal immigrants, and that genuine asylum seekers get a fair hearing.
Australia's policy of sending arrivals to Papua New Guinea and Nauru pending asylum hearings has faced criticism from UNHCR and rights groups over conditions in camps there and the lengthy process, but like previous administrations, Abbot defended it.