A major oil spill at a drilling rig off Australia's northwest coast will take at least seven weeks to contain, authorities said Monday.

The Australian Maritime Safety Authority (AMSA) said a Hercules aircraft spraying dispersant had helped reduce an oil slick that at one point stretched eight nautical miles (15 kilometres) but the spill remained a major incident.

Oil and gas began leaking from the West Atlas rig, about 250 kilometres off the Australian mainland early Friday, forcing the evacuation of 69 workers.

AMSA spokeswoman Tracey Jiggins said the Bangkok-based company that operates the rig, PTTEP Australasia, planned to bring in a second offshore platform to help cap the West Atlas well but the operation would take time.

"I don't know whether it's one of Australia's worst spills but it's certainly significant," she told AFP.

"The protracted nature of the operation means that it's a really major incident for us."

Transport Minister Anthony Albanese said seven weeks was a conservative estimate of how long it would take to cap the well-head, which lies in waters some 1.2 kilometres deep.

"This is of course is a remote location and it's a very complex operation, we need to make sure that we get it right," he told reporters.

"There unfortunately isn't a quick fix to this, we need to be honest about this."

While the government says the oil slick is not threatening Australia's pristine Kimberley coastline, green groups have criticised efforts to contain the spill.

Jiggins said AMSA responded within 15 minutes of hearing about the spill and she did not believe it had caused any environmental damage.

"We started mobilising people and resources as soon as we heard about the incident because initial indications were that it could eventuate into something significant," she said.

"We didn't want to take any risk with that so we mobilised as soon as we could and that's paid dividends."

She said two smaller aircraft were loaded with dispersant and available to back up the Hercules if needed.

"It's a day-to-day assessment, depending on conditions," she said.