Recent gale-force winds are believed to have caused the product, used in concrete slab construction, to be strewn across the landscape.

Warrnambool City Council confirmed the polystyrene was from a building site abutting the Hopkins River downstream from Jubilee Park.

Health and local laws manager Murray Murfett said it was unlikely the construction company, which he refused to name, would be fined.

"They have not acted irresponsibly at any stage," he said.

"The polystyrene blocks were blown away accidentally from the building site. I believe they are an expensive item and the incident has cost the company considerably."

The council was notified of the incident late last week.

Mr Murfett said a clean-up operation started immediately.

"They (the building company) have done an excellent job in cleaning up the blocks.

"There was also some other rubbish on the banks of the river, such as drink bottles, which they cleaned up. The (company's) staff have given up their weekend to retrieve the blocks and we really couldn't ask more than that."

Foul weather yesterday afternoon, however, forced the company to abandon the clean-up temporarily.

Mr Murfett said he hoped all the blocks would be retrieved by today.