Police have been baffled by a mysterious "unidentified" dome that washed up on a West Australian beach.

The giant metal object was found by locals at Green Head beach, about 250km (155 miles) north of Perth.

State and federal authorities are investigating the item, which is not currently believed to be from a commercial aircraft.

It is being treated as hazardous, and police have requested people keep a safe distance.

"We want to reassure the community that we are actively engaged in a collaborative effort with various State and Federal agencies to determine the object's origin and nature," police said in a statement.

These include the military and Australia's space agency.

Green Head beach residents said the cylinder was about 2.5m wide and between 2.5m and 3m long, Australia's public broadcaster reported.

Residents visited the site on Saturday night to see the cylinder, the ABC reported, with one local describing it as a "great social evening".

"It was a lovely, still night, the kids were digging sand castles around it," he told the ABC.

Aviation expert Geoffrey Thomas said the item was possibly a fuel tank from a rocket that had fallen into the Indian Ocean at some stage in the past 12 months.

The Australian Space Agency said it was possible the giant cylinder could have fallen from a "foreign space launch vehicle" and it would liaise with other international agencies.

If it is a fuel cylinder, experts believe it might be from an Indian rocket and could contain toxic materials.

It is hoped a serial or catalogue number will confirm whether or not this is the case.

There was some speculation the cylinder may have been a part of MH370 - a plane that went missing off the west Australian coast in 2014 with 239 passengers on board - but Mr Thomas said there was "no chance".

"It's not any part of a Boeing 777 and the fact is MH370 was lost nine-and-a-half years ago so it would show a great deal more wear and tear on the debris," he said.