Regular early swimmers in the harbour Guy Klamer and Alan Sacharowitz at Murray Rose Pool. Photo: Peter Rae

Most of Sydney's beaches should again be safe for swimming this weekend after the biggest storm in years on Tuesday night sent pollution levels soaring.

As of Friday, Beachwatch was predicting pollution at seven of the 25 Sydney harbour beaches it monitors, while contamination levels were also expected to be high at 13 of the 20 southern beaches tracked, with conditions likely to improve further by Saturday.

Stuart Khan,an associate professor in the UNSW School of Civil and Environmental Engineering, said simple geography meant it was likely that ocean beaches would be the first to recover, with much higher levels of water flow to flush out the pollution than those bounded by an enclosed harbour.

Beaches near the harbour are also closer to sewage and stormwater drains, the overflowing of which is the main source of contamination for inner-harbour beaches. These sites typically become significantly dirtierafter rain.

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"Those risks are real people should obey that advice not to swim in harbour pools or beaches after heavy rainfall," Dr Khan said. "It can only take between 10 and 20 millimetres [of rain]".

The city hadabout 80 millimetres ofrain in just over a day while southern suburbs were drenched in120 millimetres or more, causing minor flooding on the Cooks River flowing into Botany Bay.

Guy Klamer, a 32-year-old from Vaucluse who swims daily across a number of harbour beaches with his triathlon team the Magnesium Men, said they were mostly very clean.

"I swam at Watson's Bay after [Tuesday's] storm and it was putrid, bottles were washed up things like that," he said. "I think they're [usually] pretty clean, I can usually see to the bottom of the water, which is why I keep coming back."

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Sydney beaches safe for swimming after massive storm