The authorities have hit out at "irresponsible" ships which dumped sacks of hardened oil sludge off Johor, ruining a section of coastline.

Dozens of sacks washed up on the pristine beaches of Pulau Besar and Pulau Tengah on Friday, The Star newspaper reported, citing the southern state's tourism and environment official Freddie Long.

"We have not identified the ship that discarded the sludge yet. This is such an irresponsible act," he said.

Both islands are located off the coast of Mersing, which is the main port for ferries to the popular beach resort of Pulau Tioman and more than 40 other islands in the area.

A ferry operator who transports passengers from mainland Mersing to surrounding islands, said he first saw the sacks floating around Pulau Tengah.

"The sludge covered some two kilometres of Pulau Tengahs coastline. My passengers were shocked and disgusted to see this," he said, according to the newspaper.

A resort manager at the second affected island of Pulau Besar told The Star the sludge deposits were an annual occurrence.

He said the sludge mainly affected the uninhabited beaches facing the South China Sea and would remain until strong rain and waves washed it back to sea during the monsoon season.

A similar incident occured on Aug 6 in the town of Pontian in Johor where the state's seafood village of Kukup battled pollution when sludge flowed into open sea fish pens off the south western coast of Tanjung Piai.

Some 70 fish breeders with more than 7,500 cages were badly affected by the incident which was largely blamed on spillage from ships plying the Malacca Straits.