After a wet week across Sydney, today 73 of 160 swimming spots across Sydney were forecast to be polluted.
Earth Watch Institute chief scientist Scott Wilson said studies had shown a correlation between swimmers getting sick and heavy rainfall events when tiny but harmful bacteria could be washed into waterways.
“It’s all microbial, it’s all microscopic, so you can’t really tell necessarily,” Dr Wilson said.
The accepted safe level of bacteria in waterways is up to 40 enterococci per 100ml which pose a risk of less than one per cent of swimmers falling ill.
After heavy rain, levels of more than 10,000 are often recorded in hotspots, meaning swimmers faced a significantly higher risk.
Some of Sydney’s most popular swimming spots have recorded eye-watering levels of pathogens, up to almost 40,000 per 100ml.
The highest levels recorded this year were at Tamarama and Bronte in the city’s eastern suburbs, with 39,000 and 30,000 enterococci per 100ml respectively.
Other popular swimming spots such as Callan Park and Cabarita Beach in Sydney Harbour faced of up to 13,000 enterococci per 100ml.
Foreshores and Dolls Point Baths in the city’s south and Dee Why and Bilgola Beach in the northern Beaches all recorded between 1300 and 2000 per 100ml.
“We tend to find that our estuarine harbour beaches tend to have more pollution than those on our open coast,” Dr Wilson said.
All of the waterways are tested after heavy rain for levels of enterococci, which are measured to indicate whether pathogens are present.
“The warning usually is three days after rain, I wouldn’t be swimming before that,” Dr Wilson said.