The blaze broke out on Sunday as workers tried to plug the leak that has been spewing oil into the sea for more than two months.
PTTEP officials were planning to pump more heavy mud into a leaking well-casing on Tuesday in the hope of removing the source of fuel for the fire.
Speaking to Al Jazeera, Senator Bob Brown, leader of the Australian Green party, said the leak had been "a catastrophe for the environment" which now threatened to get even worse.
"The news goes from bad to worse. It's one of the biggest environmental catastrophes in Australian history," he said, pointing to what he said was poor government oversight of the company's efforts to tackle the leak.
The company said it did not know what started the blaze.
Oil workers had made several attempts to plug a leak more than 2.5km under the sea bed when the fire broke out.
Martin Ferguson, Australia's resources minister, said the National Offshore Petroleum Safety Authority had been asked to help.
"Some of the world's leading experts are working to fix the leaking well and respond to this latest problem," he said.
Ferguson said that once the spill was contained, he would launch an inquiry and, if PTTEP was "found to have been at fault with respect to any of their responsibilities, then any potential action will be appropriately considered at the time".
The rig began leaking oil on August 21 spewing between 400 and 2,000 barrels of crude into the Timor Sea each day.
It had been due to start commercial operations this month.
Environmentalists estimate that about 10 million litres of oil have emptied into the sea so far, saying the leak had seriously harmed wildlife and that its effects could be felt for 20 years.
They have said that efforts to clean up the slick, using chemical dispersants, could have an equally damaging effect on wildlife.
An Australian government report said birds and marine species were at risk from the oil spill, but added that the full impact could not be immediately determined.