Thai-owned PTTEP Australasia was in March found to have breached its duty of care to thousands of Indonesian seaweed farmers after more than 180,000 barrels of oil gushed into the Timor Sea in 2009.

Crops in the Rote/Kupang region were wiped out, impacting farmers whose income is estimated by one expert to be relied on by more than half of the region's households.

The Federal Court on Monday settled a dispute between the farmers and the oil company as to what parts of the region were touched by the oil spill.

The region's land area resembles that of Geelong or the NSW Central Coast and is home to more than 150,000 people.

PTTEP contended the court could not make a finding beyond a few dozen specific locations where individual farmers or community members had told the court they'd witnessed seeing oil or seaweed damage.

That was based on its evidence the oil moved in lines and "tigers tails", not waves.

The farmers' lawyers argued the witnesses' evidence could be used to infer the Montana oil spill reached and caused damage to seaweed at all coastal islands in the Rote/Kupang region.

The judge found it was likely the oil was widespread and present around the coastal areas from September to at least November 2009.

"It would defy common sense to think that oil from the H1 Well blowout reached only the locations (referenced) and no other locations in the coastal areas of Rote/Kupang," Justice David Yates said on Monday.

The spill also likely caused, or at the very least materially contributed to, the "quick and dramatic loss" of local seaweed crops, he said.

But the judge stopped short of finding damage to every seaweed farm in the area over the relevant time period was caused by oil as the court hadn't heard from farmers in all villages.

The decisions bring the parties ever closer to settling the class action, initiated in 2016 on behalf of 15,000 farmers.

Lead litigant Daniel Aristabulus Sanda, who lived on about $2000 a year before taking up seaweed farming on Rote Island, was awarded 245 million Indonesian rupiah ($A24,000) plus interest for losses over a six-year period.

PTTEP had told authorities and the court the oil was spilling at no more than 400 barrels per day.

But the court found it gushed at an uncontrolled rate in excess of 2500 barrels a day - with a very large part of the spilled oil not treated with dispersants.

PTTEP accepted it was negligent in suspending and operating the well but disputed it had a duty of care to farmers who it said hadn't proved the oil was in a form that was toxic to the seaweed crops.

A seaweed farming expert, marine biologist Iain Neish, told the trial Indonesia was now the world's dominant source of tropical seaweeds and the industry was an extremely important livelihood for communities in the Rote/Kupang region.

Australian Associated Press