The West Atlas oil rig after a fire on it was put out 150 miles off Australia's north-west coast (AP Photo/PTTEP Australasia)

Australian authorities have launched an investigation into an oil rig leak that dumped thousands of barrels of fuel into the Timor Sea over more than two months before being plugged this week.

The government inquiry will focus on the cause of the leak, the response by rig operator PTTEP Australasia and other officials and the spill's impact on the environment.

An estimated 400 barrels of oil spilled into the Timor Sea each day since the leak began on August 21.

On Sunday, a fire broke out at the West Atlas rig and Montara wellhead platform as workers were pouring mud into the leaking hole. Officials finally managed to plug the leak and extinguish the fire on Tuesday.

The company, a unit of Thailand's PTT Exploration & Production Plc, says it does not know what sparked the blaze.

Former environmental department secretary, David Borthwick, will head the inquiry, the government said.

"I simply say that we aspire to learn from this incident and take any necessary steps to stop similar incidents occurring in the future," federal resources minister Martin Ferguson said.

The oil slick from the rig, about 150 miles (250 kilometers) off Australia's northwest coast, now stretches across thousands of miles (kilometers) of remote ocean.

PTTEP said this week it would cooperate with the inquiry.

Borthwick's report is due in April.