AAP Some of Sydney's prime swimming spots have had their water quality ratings downgraded.

Pollution on Sydney's beaches is not due to job cuts or untreated waste water being pumped into the ocean, Sydney Water says.

The latest State of the Beaches report released on Sunday found that the water quality at formerly top-ranking beaches has dropped over the past year.

While 81 per cent of swimming locations reviewed across the state achieved very good or good gradings, only eight Sydney beaches are rated as very good - down from 15 last year.

The NSW opposition blamed the drop in water quality on job and budget cuts at Sydney Water.

It also pointed to a record number of bypasses at waste-water treatment plants.

"We are returning to the bad old days when it was unsafe to swim on the beaches due to overflows," opposition water spokesman Walt Secord said in a statement.

"The O'Farrell government cannot blame rain as NSW has experienced its warmest January to September period on record. NSW has had its mildest winter in more than 150 years."

But Sydney Water rejected the comments, saying they were untrue.

"The Beachwatch 2013 Report released today shows that 37 out of 38 ocean beaches in Sydney were graded as good or very good, despite a wet summer in 2012/13, which is an outstanding result," Sydney Water said in a statement.

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Sydney's cleanest beaches downgraded