Law firm Maurice Blackburn is seeking more than $200 million in compensation for Indonesian seaweed farmers from Australia's worst oil spill, in the Timor Sea in August 2009.

The Montara oil spill was caused by a huge explosion, also known as a blowout, at an oil well drilled by a subsidiary of state-owned Thai oil firm PTT Exploration and Production Public Company (PTTEP).

Maurice Blackburn principal and NSW manager Ben Slade said more than 13,000 Indonesian farmers are party to the class action in the Federal Court in Sydney.

The law firm is suing PTTEP Australasia (Ashmore Cartier) Pty Ltd, a subsidiary of PTTEP.

Mr Slade said the Indonesian farmers are claiming damages from the environmental disaster which devastated their livelihood.

"They are claiming compensation for the loss and suffering caused by the negligence of the oil rig operator," he explained.

"We say that runs into hundreds of millions of dollars. Our assessment to date is something north of $200 million that they are entitled to."

'Things were going really well' for farmers until spill

Indonesian seaweed farmer Daniel Sanda will be the lead plantiff in the case. Mr Sanda is a subsistence farmer who lives on the shores of Rote Island.

Mr Slade said PTTEP Australasia had a duty of care towards the farmers.

"Things were going really well for their community and, in fact, they had a bumper year in 2008 but towards the end of 2009 everything stopped and their crops died," he said.

"It was really quite a horrific incident and the company has for many years now tried to avoid compensating the Indonesians who suffered so greatly.

They have taken many years to get back to any sense of normal production.

Mr Slade said the legal team would be relying on official records from Indonesian authorities for evidence, although he conceded the case will be challenging.

"This case is replete with many problems. The analysis of the losses is difficult, but there are records, people paid tax," he observed.

"The records are there, it's just a big job to get it together."

10 swimming pools of oil a day gushed for 10 weeks

The oil spill occurred in Australian waters, nearly 700 kilometres west of Darwin and 250 kilometres away from Rote Island in southern Indonesia.

It took just over 10 weeks for the massive oil leak to be plugged and an estimated 300,000 litres of oil gushed into the Timor Sea each day.

That is equivalent to 10 Olympic swimming pools of oil over the 10 weeks.

PTTEP Australasia operates the Montara oil field and the Cash Maple gas condensate field in the Timor Sea, employing more than 200 people in Perth, Darwin and the Timor Sea.

PTTEP is publicly listed on the Thai stock exchange and operates more than 40 projects globally with 4,000 workers.

In 2013, four years after the spill, PTTEP Australasia said the Timor Sea had been given "a clean bill of health" by the Montara Environmental Monitoring Program, which was developed by PTTEP and the Australian Government.

Oil spill 'worst of its kind' in Australian history

A commission of inquiry set up to investigate the Montara oil spill described the disaster as "the worst of its kind in Australia's offshore petroleum industry."

The inquiry found that PTTEP Australasia (PTPAA) did not observe sensible oilfield practices at the Montara Oil field.

"Major shortcomings company's procedures were widespread and systemic, directly leading to the blowout," the inquiry said.

The inquiry also criticised the Northern Territory Department of Resources (NT DOR) as "not a sufficiently diligent regulator."

"The way the regulator (the NT DOR) conducted its responsibilities gave it little chance of discovering PTPAA's poor practices," the Montara Commission of Inquiry report noted.