More than 13,000 Indonesian seaweed farmers are launching a $200m class action against the company they claim was responsible for Australia’s worst offshore oil spill, which sent millions of litres of oil towards their shores.

The action to be filed in the federal court in Sydney on Wednesday follows a seven-year fight for justice by Timor Sea communities, who say their lives changed dramatically for the worse after the 2009 Montara disaster.

Maurice Blackburn Lawyers’ class actions principal, Ben Slade, said the Montara rig operator, PTTEP Australasia, must be held accountable for the avoidable oil spill and the devastation it had caused Australia’s neighbours.

“The operator of the oil rig has a serious case to answer for cutting corners that endangered lives, the environment and the livelihoods of thousands of seaweed farmers,” he said in a statement.

The farmers from the islands of West Timor and Rote were making a decent living from growing seaweed for cosmetics and other industries, but saw production plummet soon after the 74-day spill.

The lead plaintiff, Daniel Sanda, 58, said that in September 2009, he noticed oil and a large number of dead fish in the waters around his seaweed crop at Oenggaut, a village in Rote.

It spelled disaster for his family.

“If, on the first day you noticed oil on the water, by the next day your seaweed would turn white, and the next day it would be totally dead,” he said. “It was completely confusing to us and very hard for me.

“I had to think about my family, and it became so hard to live a normal life.”

Seaweed had been a boon for villagers like Sanda, with the income from the healthy harvests allowing him to send three of his five children to university.

The industry hasn’t bounced back and Sanda hopes for compensation for years of lost income.

“I just hope it will succeed,” he said. “Until this day, we can’t grow seaweed as successfully as before.”

Josias Ferroh was one of the pioneers of seaweed farming on Rote.

Several years of successful harvests allowed him to make modest improvements to his simple home, and send his eldest child to university.

Like Sanda, in September 2009 his seaweed began to die.

“It happened so suddenly, I thought it must have been a disease,” Ferroh said.

The effects have not only been economic, but some people in West Timor and Rote have reported skin lesions and respiratory problems after coming into contact with water.

Community advocate Ferdi Tanoni has spent years campaigning for the Australian and Indonesian governments to launch an independent environmental study in the region.

In 2014, the Indonesian government directly requested cooperation from Canberra, which said it did not have jurisdiction.

Tanoni said there was a sense of relief in West Timor that there was finally action.

“It is now seven years, less 18 days, since the spill,” he said. “For all that time, these people have been waiting for justice.”

PTTEP Australasia’s own study found no oil had reached Indonesia’s coast or inshore waters. The company has been contacted for comment.

In 2012, a Darwin court fined the company $510,000 for the spill.

It began on August 21, 2009, with a dramatic explosion and fire on the West Atlas rig, 250km off Australia’s coast.

An estimated 300,000 litres of oil a day spewed into the sea until the leak was plugged on 3 November 2009.

The federal government held a commission of inquiry into the event, and no evidence was found of long-term damage to Australia’s reefs and coasts.

A PTTEP Australasia spokesman said the company had always accepted responsibility for the spill.

“While PTTEP Australasia believes the class action filed in Australia on behalf of the communities of Nusa Tenggara Timor is misguided, we nevertheless respect their right to lodge this claim,” a statement said.

The company stood by the research it had ordered in the wake of the incident.

The satellite imagery, aerial surveys and trajectory modelling found the majority of oil (98%) remained in Australian waters, it said.

“We are confident the results of these independent studies would stand up to the highest scrutiny,” the statement said.

“As would our assertion that it is reasonable to extrapolate from the studies that, if the reefs closest to Montara where the oil and dispersant concentrations were at their greatest did not show lasting impacts, then it is highly improbable that the seas and coastline of Nusa Tenggara Timor would have been impacted.”