The Southeast Asian nation is one of the world's biggest contributors of plastic pollution and marine debris, with annual monsoon rains and winds sweeping mountains of plastic waste from its cities and rivers into the ocean.

Some of it drifts hundreds of kilometres before washing up on the beaches on the holiday island -- especially between November and March.

Across Kedonganan beach in the south of the island, plastic cups, straws, cutlery, and empty coffee sachets were scattered across the sand, mixed with plant and wood debris.

- Tons of garbage -

Around 600 volunteers, including local residents, hospitality workers, and tourists, braved a rainy morning to pick up the waste by hand before filling hundreds of large sacks.

The Environmental NGO Sungai Watch called it "the worst" plastic waste pollution to wash ashore in Bali.

"We have never seen plastics a metre thick in the sand. In just six days of cleanup, we collected 25 tons, which is a record for us," said Sungai Watch founder Gary Bencheghib.

Bencheghib said an audit found most of the plastic waste came from cities on neighbouring Java, Indonesia's most-populated island.

Tatiana Komelova, a Russian tourist volunteer, said the sight of the pollution shocked her, and motivated her to reduce the use of plastic in her daily life.

"I knew the problem existed, but I didn't know it was this bad," she said.

"I use plastic products a lot in my life, and now I try to reduce it as much as possible."