Foreshore Beach at Port Botany is Sydney's most polluted, according to a report ranking the cleanliness of NSW beaches and swimming sites, but swimming spots around the state are generally becoming cleaner.

The report by the Department of Environment's Beachwatch water quality monitoring service examined 94 of Sydney's beaches and estuarine swimming sites. About 90 per cent were ranked "good" or "very good".

The orange plume that invaded Foreshore Beach at Port Botany in February. Credit:NSW EPA

Eight were found to be "poor". And only Foreshore Beach, which is next to Sydney Airport's third runway, received a "very poor" rating - for the second year running.

Foreshore was closed earlier this year after concerns were raised after the ocean took on a mysterious orange tint.