As Sydney prepares to swelter, the city's cleanest beaches have been revealed — and there are two local government areas dominating the list.

The NSW environment department's annual Beachwatch report analysed 249 sites across the state, measuring microbial water quality across four grades from "very good" to "very poor".

At least five swimming spots in the Northern Beaches area received a "very good" grading for water quality, which meant they were suitable for swimming almost all of the time.

The beaches included Avalon, Palm, Bungan, Bilgola and Whale Beach, which claimed the top spot for cleanliness after being tested for faecal matter and other pollutants.

In Sydney's south, Greenhills, Wanda, Shelly, Elouera and North Cronulla beaches in the Sutherland Shire fared equally as well,

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