SYDNEY - The company at the centre of a massive oil spill off Australia's northwest said Tuesday it was still about two weeks away from plugging the leak, which has already been gushing for more than a month.

An estimated 400 barrels of oil has spilled daily into the Timor Sea since the West Atlas drilling rig began leaking on August 21, forcing the evacuation of 69 workers, according to Bangkok-headquartered PTTEP Australasia.

PTTEP said it began drilling a relief well to divert the leaking oil and gas on Monday, after a mobile rig arrived from Indonesia. It needs to bore 2.6 kilometres (1.6 miles) under the seabed, which will take about a week.

"This will be followed some 24 hours later by a specialist crew boarding the West Atlas rig and well head platform to further secure the situation by plugging the well bore," the company said in a statement.

"This operation is expected to take about another week."

Heavy mud would be used to plug the leaking bore, it said, adding that more than 300 people were involved in the relief effort about 250 kilometres off the Australian coast.

Tonnes of dispersant chemicals have been dumped on the spill, reportedly Australia's worst since offshore drilling began more than 40 years ago, and ecologists fear the toxic cocktail could threaten marine and coastal species.