The cleanest and dirtiest beaches in Sydney revealed as Harbour City looks to cool off during summer

The NSW government has revealed the cleanest and dirtiest beaches to swim at in Sydney, as the Harbour City prepares for a hot summer.

Temperature records are already being broken, leading Sydney's residents to flock to the 97 beaches around the city to beat the heat.

The latest BeachWatch State of the Beaches report reveals the Northern Beaches have the cleanest water in Sydney, with all but one of the beaches in the top 10 being in the area.

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Several factors are used to measure the water quality.

One of these is measuring the level of a bacteria called enterococci.

It is an indicator of germs present in water that can cause gastrointestinal illnesses, as well as rashes.

Swimming in beaches and waterways is considered safe when the level of the bacteria is less than 41 per 100 millilitres of water.

According to this year's figures, Whale Beach on the Northern Beaches has the cleanest water in the city, having just 5 enterococci/100mL of water, well below the unsafe limit.

Most of the top 10 cleanest beaches in Sydney are in the area, with Shelly Beach in the Sutherland Shire being the only beach outside the area to make the cut.

On the other end of the scale, Foreshore Beach near Sydney Airport had the worst rating, with 1150 enterococci/100mL of water detected.

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NSW Health gave the beach an annual grade of "very poor", explaining there was a 10 per cent risk of picking up a gastrointestinal illness when at the beach and a nearly four per cent chance of fever or rash.

Several of the beaches and swimming areas with very poor or poor overall grades are in the southern part of the city.

Many of these are not beaches but estuaries or lagoons.

Just four locations in the Northern Beaches had this overall grade, with only one - Narrabeen Lagoon - being in the 10 dirtiest beaches in Sydney.

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The city's most famous beaches, including Bondi and Coogee Beach, received mixed ratings.

Bondi Beach received a good grading despite its microbial assessment indicating it had more harmful bacteria than the recommended limit, with Bronte Beach and Tamarama also having similar gradings.

Coogee Beach was given a poor grading, with 260 enterococci/100mL of water, meaning swimmers have a 5-10 per cent chance of contracting a gastro-like illness.

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In a statement unveiling this year's report, health experts said the data showed an overall improvement in the quality of water at Sydney's beaches.

This is because there has been less than average rainfall, which can cause more pollution in waterways.

Swimmers are advised not to enter the water when it is raining and for three days after, when contaminants and debris still could remain.

"This year we have seen a great result, and we will be continuing to work hand-in-hand with councils to ensure the program continues to serve the community," NSW Climate Change and Energy Minister Penny Sharpe said.

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