Dozens of workers have been evacuated from a Timor Sea oil rig off Australia after an oil and gas spill, rig operator Seadrill said.

A national clean-up plan has been activated to deal with the incident on the offshore drilling rig at the Montara development, about 250km off the far north Kimberley coast and 150km south-east of the Ashmore Reef, according to the company.

PTTEP Australasia said the oil and gas spill happened at about 4am (WST) on the West Atlas rig, operated by Norway’s Seadrill, writes the Sydney Morning Herald.

None of the 69 people on board the rig was injured in the incident and all were evacuated, according to PTTEP.

About 40 barrels of oil are believed to have spilt in the initial incident with condensate also being discharged along with gas.

The extent of the spill and environmental damage caused has not been established.

The Australian Marine Oil Spill Centre has sent aircraft to Darwin, carrying chemicals to mop up crude oil, writes the newspaper.