The slick, which is up to 30km long, yesterday responded well to bombardment with chemical dispersants from a RAAF C130 Hercules, and appeared to be contained near the damaged West Atlas drilling unit, 250km northwest of Truscott air base in the Kimberley region.

West Australian Premier Colin Barnett said it appeared unlikely the spill would move towards the Kimberley coast, but an emergency management plan was in place to further disperse the oil and recover wildlife if necessary.

On Friday, 60 workers were evacuated from the rig after sweet light crude oil, gas and condensate started to seep through a concrete and rubber plug at the end of the well pipe, 2.6km below the floor of the ocean.

One of the companies operating West Atlas, PTTEP Australasia, still does not know how the sub-surface leak occurred or how much oil and gas is flowing out of the wellhead.

The Hercules yesterday flew five runs up and down the slick, dropping tens of thousand of litres of dispersants.

The Australian Maritime Safety Authority said the dispersant was breaking up the oil into smaller globules and scattering it throughout the water.

An AMSA spokeswoman said the spill was being contained close to the rig but because it was breaking up it was difficult to assess its size.

Mr Barnett said he believed the slick was drifting away from the mainland in a northwesterly direction.

He said two fixed-wing aircraft were ready at Truscott with more dispersants in case the weather changed and some of the spill headed towards the coast.

That did not appear likely and the oil appeared to be evaporating rapidly.

"Most of what is escaping is gas, which finds its way into the atmosphere, and the oil tends to be a light crude or condensate, which breaks up and evaporates," Mr Barnett said. "So it's not damaging in this sense. The immediate concern is the Ashmore Reef area, which is acommonwealth jurisdiction."

The authority said the slick was not approaching Ashmore Reef.

Green groups called for all gas and oil development and exploration, including the Gorgon LNG project, to cease following the spill.

Greens leader Bob Brown said there should be a judicial inquiry into how the oil rig was run, and the clean-up operation.

The Australian Petroleum Production and Exploration Association defended the safety of the oil and gas industry.

"This is very unusual, this incident. We haven't actually had an offshore gas well blowout since 1984," deputy chief executive Mark McCallum said. "Not in over 1500 wells have we had a gas well blowout."

The association said it would take about 50 days to fix the leak.

It plans to tow the new rig to a point at least 2km off West Atlas, where a team will drill a relief well over 20 days.

They will use magnetic equipment to guide the drill towards the original pipeline to intersect it and spend a further eight days injecting it with heavy mud to stop the leak.

Federal Infrastructure Minister Anthony Albanese would not say how much the clean-up could cost. "The concern that the government has isn't the cost of the clean-up," he said.

"We want to make sure that it's got right, and that no corners will be cut, to ensure that maximum effort is made to minimise any impact of this oil spill."