State of the Beaches 2016-17, which monitors the water quality of 250 NSW swimming sites, found 84 per cent of those tested were rated good or very good, a slight improvement on 2015-16.

But the water quality of Sydney's beaches declined slightly over the same period, due to heavy rainfall in February and March, which washed pollutants into the water.

The Beachwatch report found 84 per cent of the 97 Sydney swimming sites tested had either good or very good water quality in 2016-17, down from 86 per cent on the previous year.

Despite the drop, Sydney's beaches are much cleaner now than in the 1990s, NSW Environment Minister Gabrielle Upton said.

"Beachwatch's water quality monitoring shows that Sydney's beach water quality has improved dramatically over the last two decades," she said.

Persistent storms lashed the city in February and March, making it the wettest since 1975 and washing rubbish and other pollutants into the water.

"Heavy rainfall generates stormwater, a major cause of pollution to recreational waters," Ms Upton said.

"Councils and water authorities are constantly investing in improving water quality. It's important to remember whatever is thrown on the ground may end up in stormwater and eventually lead into waterways."

The report found ocean beaches are significantly cleaner than lagoons or estuarine beaches.

Sydney's most polluted swimming spot is Foreshore Beach, a small strip of sand between Sydney Airport and Port Botany. It was the only Sydney beach to receive a very poor rating.

Sites that received a poor rating include some of Sydney's most popular swimming spots such as Narrabeen Lagoon and Barrenjoey Beach in the north, Rose Bay Beach, Gymea Bay Baths and Oatley Bay Baths.

The only ocean beach to receive a poor rating was Malabar.

Three swimming spots went up in the rankings, including Coogee Beach, South Cronulla and the Gunnamatta Bay Baths.

Seven areas declined in water quality, including Nielsen Park and Rose Bay Beach in the east, Tambourine Bay near Lane Cove, and Oatley Bay Baths, Carss Point Baths, Congwong Bay and Horderns Beach in the south.

Sydney's north is the cleanest area for a dip with 91 per cent of sites rated good or very good.

The report found 81 per cent of swimming spots in central Sydney, defined as harbour areas and ocean beaches from Bondi to Little Bay, were good or very good.

Seventy-nine per cent of southern sites, which included Sutherland beaches, lower Georges River, Botany Bay and Port Hacking were ranked good or very good.