PORT MORESBY: Papua New Guinea ordered the snap closure of borders used by Indigenous groups to cross to and from Australia Thursday, as fears grew about the spread of Covid-19 to remote communities.

Police Commissioner David Manning said travel between small islands that straddle the two countries' maritime border in the Torres Strait would be closed "with immediate effect."

Under a 1978 agreement, Indigenous inhabitants of the islands can pass between Australia and Papua New Guinea without passports or visas.

Torres Strait Islanders have a culture that is distinct from Aboriginal Australians and Papua New Guinea's myriad of other tribal and linguistic groups.

The announcement comes as Papua New Guinea struggles to contain a wave of coronavirus infections, which officials fear could enter largely Covid-free Australia and decimate vulnerable Indigenous communities in the far north of the country.

"Those borders will remain closed for an indefinite period, and cross-border travel will not be allowed until further notice," said Manning, who is leading Papua New Guinea's Covid-19 response effort.

On Wednesday, Papua New Guinea reported a record of 351 new Covid-19 cases, up from almost no cases a few weeks ago.

Since the pandemic began, the country has conducted around 60,000 tests, detecting 4,109 cases.

But a rapid rise in infections is already threatening to overwhelm the country's already under-resourced health system, with patients reportedly being treated on the floor of hospitals in the capital Port Moresby.

International air and sea travel to Papua New Guinea had already been subject to Covid restrictions and the land border with Indonesia is closed.