NSW beaches are among the cleanest in the world according to a new report which also found that the water quality at some of Sydney's favourite swimming spots has declined.

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.