A massive oil spill off northwest Australia has hit Indonesia, killing fish and destroying livelihoods in poor villages along the Timor Sea, a non-governmental group said Monday.

About 7,000 fishermen in East Nusa Tenggara province have been affected by the spill from the Thai-operated West Atlas rig, which has dumped thousands of barrels of oil into the Timor Sea since August 21, the group said.

"After the leak started the fishermen's income dropped 40 percent but since last week it's been 80 percent lower," said Ferdi Tanoni of the West Timor Care Foundation, which supports poor fishermen in eastern Indonesia.

"Before this incident, they were able to catch about 100 red snapper a night. But now it's extremely difficult to even get 20 fish."

The PTTEP Australasia-operated rig caught fire Sunday during an attempt to stop the leak, engulfing the deck and well-head platform some 250 kilometres (155 miles) off the Australian coast.

Environmental group WWF has said the spill is "one of Australia's biggest environmental disasters".

It says more than 400,000 litres (over 105,670 gallons) of oil have been spilt, generating a slick spanning 10,000-25,000 square kilometres (up to 9,650 square miles) that threatens animals including dolphins and sea turtles.