Several contaminated ocean pools, baths and lakes across Sydney were found in samples taken in November and January, according to the test commissioned by the Sydney Morning Herald.

At Dawn Fraser Baths in Balmain, faecal contamination was 10 times above safe levels at the time of testing.

Mort Bay in Birchgrove was five times higher than safe levels and results at Double Bay’s Redleaf were double what they should have been.

Nielsen Park, Murray Rose pool and Barangaroo were also polluted with elevated levels of faecal matter.

Samples from Balmoral Beach, Penrith Beach, Windsor Beach and Lake Parramatta were given the all clear.

Western Sydney University PhD candidate Katherine Warwick, who was commissioned to test the sites, said she found a bacteria known as enterococci that can cause gastrointestinal illness.

“It’s sometimes not that much of a surprise because the quality of the water can change almost hourly,” Warwick said.

“It’s really quite fickle, but what this does tell us is how quickly it can change.

“If you are swimming in that area and you do become sick just after swimming there, chance are you’ve ingested that bacteria and [should] get yourself to a health professional immediately.”

It’s understood the ocean’s dirty secret is caused by a culmination of run-off from construction sites, dog poo on the street and sewage overflows.

Pollution monitoring program Beachwatch advises people where and when to swim, but usually only tests once a week.

“So it takes 24 to 48 hours to get a result back. And so, for some people, they may not know until they’re already sick that they swam in a place that wasn’t safe,” Warwick said.

Inner West Mayor Darcy Byrne is adamant Dawn Fraser Baths is safe, but is calling for more frequent testing to ensure the public that it’s safe to swim.

“But the fact that there was one particular day where there was this concerning sample means we have to investigate,” Byrne said.

“And I think if we have to do testing more frequently to give the public the confidence to swim here safely, that’s what should happen.”

Swimmers are advised to check Beachwatch and not to go swimming after rain.

Regardless, swimmers at Dawn Fraser today were simply not bothered.

“[The] water seems fairly clean, people have been swimming it for hundreds of years,” one swimmer said.