More than 15,000 Indonesian seaweed farmers have launched a Sydney lawsuit claiming a 2009 oil spill in Northern Australia destroyed their livelihoods.

The class action, led by Daniel Sanda, is against the oil company PTTEP Australasia (Ashmore Cartier) Pty Ltd and relates to the Montara oil spill that resulted in thousands of barrels of oil pouring into the Timor Sea over 70 days.

Opening the case in the Federal Court on Monday, their barrister Julian Sexton SC alleged the company was negligent in failing to safely operate the Montara wellhead and the incident had been foreseeable.

The mixture of the oil and chemical dispersants, used after the spill, led to the solution spreading and destroying the seaweed crop and industry for that year and many years after, he said.

AAP Image/PTTEP

The oil field was located about 250km northwest of the West Australian coast and about 700km from Darwin.

The rig spewed oil and gas into the Timor Sea, in Australian waters 690km west of Darwin and 250km southeast of Rote Island, Nusa Tenggara Timur, Indonesia.

"There were a number of intersecting acts and omissions, each of which was negligent," Mr Sexton said.

The blowout on the oil rig occurred on 21 August in 2009, resulting in the 69 workers being evacuated and an exclusion zone created.

Four sealing attempts failed, but the fifth try was successful after 74 days on 3 November "not without some drama because a well ignited", he said.

Mr Sexton said there was some debate about how much oil was released during that time, but PTTEP had never publicly identified the basis of its estimate of 200 to 400 barrels a day.

"There was no contemporaneous scientific measure of the quantity of oil released," he said.

But as a result of the spill, oil or sheen was observed over an area of approximately 90,000 square kilometres, and seaweed cultivated by the farmers was killed or destroyed.

As well as the immediate loss of seaweed, they suffered a later drop in production.

Earlier, a lawyer from Maurice Blackburn, Ben Slade, told journalists the case was set down for 10 weeks.

Witnesses would include seaweed farmers who saw the oil arrive and destroy their crops as well as experts on oil spill modelling, chemistry and environmental impact.

Mr Sexton will continue his address on Tuesday before Justice David Yates, followed by an opening by the barrister for PTTEP which is fighting the action.