Police say they are not sure how long it will take them to find a man who fell into a sewage pond near Dandenong in Victoria.

A laboratory technician in his 50s slipped into the pond at the Eastern Treatment Plant at Bengholme just before 9am AEDT while he was taking a sample.

More than 1.5 million litres of sewage was pumped out of the ponds, with the the Country Fire Authority, police and specialist divers spending eight hours trying to find the man.

Senior Sergeant Pat McGavigan says it is a difficult situation.

"I can't give you a time, it'll take as long as it takes to do it safely," he said.

"Search and rescue are in charge of the recovery section at the moment and whatever they require we will obtain and utilise their services."

Some of the man's personal equipment was found next to the pond.

The plant treats about 40 per cent of Melbourne's sewage or about 312 million litres a day.