A dead whale that washed ashore had a large lump of plastic waste in its stomach, including drinking cups and flip-flops.

The "awful" discovery in eastern Indonesia has caused concern among environmentalists and government officials in one of the world's largest plastic polluting countries.

Rescuers from Wakatobi National Park found the rotting carcass of the 9.5-metre sperm whale on Monday in Southeast Sulawesi after hearing villagers had surrounded the dead whale and were butchering the rotting carcass.

Park chief Heri Santoso said researchers from wildlife conservation group WWF and the park's conservation academy found about 5.9kgs of plastic waste in the animal's stomach.

A dead whale filled with plastic has washed up on an Indonesian beach. (AP)

This included 115 plastic cups, four plastic bottles, 25 plastic bags, two flip-flops, a nylon sack and more than 1,000 other assorted pieces of plastic.

"Although we have not been able to deduce the cause of death, the facts that we see are truly awful," said Dwi Suprapti, a marine species conservation coordinator at WWF Indonesia.

It wasn't possible to determine if the plastic was responsible for the death because of the advanced state of decay.

It was filled with 115 plastic cups, four plastic bottles, 25 plastic bags, two thongs, a nylon sack and more than 1,000 other assorted pieces of plastic. (AP)

Indonesia, an archipelago of 260 million people, is the world's second-largest plastic polluter after China, according to a study published in the journal Science in January.

It produces 3.2 million tons of mismanaged plastic waste a year, of which 1.29 million tons ends up in the ocean.

Luhut Binsar Pandjaitan, Indonesia's coordinating minister of maritime affairs, said the whale's discovery should raise public awareness about the need to reduce plastic use.

"I'm so sad to hear this," said Pandjaitan, who recently has campaigned for less use of plastic.

Rescuers from Wakatobi National Park found the rotting carcass of the 9.5-metre sperm whale on Monday in Southeast Sulawesi province. (AP)

"It is possible that many other marine animals are also contaminated with plastic waste and this is very dangerous for our lives."

Indonesia is trying to reduce the use of plastic, by urging shops not to provide plastic bags for customers and teaching about the problem in schools, to meet a government target to reduce its use by 70 per cent by 2025.

"This big ambition can be achieved if people learn to understand that plastic waste is a common enemy," Pandjaitan told The Associated Press.