Australia denied claims Saturday it had downplayed the scale of a massive oil spill at a drilling rig off its northwest coast, and said the slick was dispersing naturally.

Resources Minister Martin Ferguson said the Greens party was exaggerating the size of the leak at the West Atlas drilling platform, about 250 kilometres off the Australian mainland.

"The area of the spill is rectangular in nature," said Ferguson.

"It is to the north-east of the rig and 15 nautical miles (28 kilometres) to the north and 60 nautical miles to the east.

"Contrary to what the Greens are suggesting, the closest it is to the Australian coastline is in excess of 80 miles (129 kilometres)," he added.

Ferguson's comments came after Greens Senator Rachel Siewert flew over the spill in a chartered plane Friday and said it was "far bigger than we have been told and closer to the coast than expected."

"From east to west it stretches 180 kilometres (112 miles) at a minimum. Urgent action is needed to stop the flow," she added.

Tonnes of dispersant chemicals have been dumped on the slick, and Ferguson said most of it was breaking up naturally.

"This spill will continue to spread in a north-easterly direction and I must say, as of today the weather conditions are assisting," Ferguson said.

"They are a bit more choppy and that will assist in the natural break-up of the oil and gas."

The leak began early last Friday at the West Atlas rig, forcing the evacuation of 69 workers.

Its Bangkok-based operator PTTEP Australasia was unable to cap the leak, and authorities have warned it could take up to seven weeks to contain, with a second rig sent from Singapore for the repair operation.

Based on average flow rates in the region and data from the company, Siewert said almost half a million litres (132,000 US gallons) of oil was daily spilling into the ocean.

She urged PTTEP and Norway's Seadrill, which owns the West Atlas platform, to accept the offer of a relief rig from Australian company Woodside Petroleum, which could be on the site within five days.

But PTTEP said there was no indication Woodside's equipment would reach West Atlas any faster than the Singapore rig, which it estimated would arrive in "seven or eight days".

"The other thing is there's different types of rigs and the option that has been selected by the company is the safest, most effective and most likely for success," said PTTEP spokesman Ian Williams.

He would not comment on Siewert's claims about the size and extent of the spill.

PTTEP on Monday estimated the slick was eight nautical miles long and 30 metres wide, and said it had "not shown signs of expanding".

It plans to drill a relief well with the secondary rig to intersect the leaking well head and stop the flow of oil and gas.

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