Scientists say there is a clear east-versus-west divide that is leaving western Sydney residents at risk of swimming in contaminated water this summer.

Key points: Beachwatch publishes data on water quality at Sydney's eastern beaches and some harbour locations

Beachwatch publishes data on water quality at Sydney's eastern beaches and some harbour locations There's no publicly available data on swimming spots west of Cabarita, although some councils tests water quality

There's no publicly available data on swimming spots west of Cabarita, although some councils tests water quality The Office of Environment and Heritage says it has no plans to extend its Beachwatch program

As temperatures in parts of the west reached 42 degrees Celsius by midday yesterday, hundreds flocked to the Hawkesbury, Nepean and Parramatta rivers — but had no way of knowing if the water was safe.

The New South Wales Government's Beachwatch conducts regular testing of Sydney's eastern beaches and some harbour locations, but does not test at any swimming spots west of Cabarita.

Ian Wright, who specialises in water pollution at Western Sydney University, said there was no public data on the water quality at Windsor Beach on the Hawkesbury River — an omission which he said was comparable to having no data for Bondi Beach.

"This is the Bondi Beach of western Sydney," he said.

"This is incredibly popular but I can't give you any guidance on water quality and the risks involved.

"I don't know how swimmable that is."

Dr Wright said the brown water "doesn't look great" and there were many risks to human health due to the microbiology of the river.

"Historically there have been so many water quality issues here," he said.

Yarramundi Reserve is where the Grose and Hawkesbury-Nepean Rivers merge and is a popular spot for a dip. ( ABC News: Lily Mayers )

"It has high nutrients, it has blue-green algae from time to time, water weeds, and it also has the likely presence of human pathogens, so bacterial contamination is also an issue here."

"It's a huge act of faith to jump into this water."

Dr Wright said when local councils tested the water, the results were not made public.

"Why do we have so little information available? Yet we have so much development about to hit here and every bit of development will have so many more water quality issues associated with it," he said.

Ian Wright tests the quality of the water at Windsor Beach. ( ABC News )

"If the State Government says that Beachwatch is the responsible authority in the east and the harbour, why isn't it also the responsible authority in the west?"

Around 20 to 40 per cent of the normal river flow in the Hawkesbury River in dry weather is treated sewage, according to Dr Wright.

"This is where a lot of Sydney's drinking water comes from further upstream and yet this is where so many of the pollution issues from runoff, agriculture and sewerage hit."

He isn't the only scientist concerned by the lack of data available for the west.

Stuart Khan from the University of New South Wales said there should be a focus on ensuring the upper Parramatta and Nepean rivers were safe for swimming "because they should be, they're jewels of Sydney".

Dr Khan also called on the Beachwatch program to be extended.

"It's a question of funding, it's a question of getting the State Government to come onboard and say that this is an important enough priority for us to be investing funds in."

No plans to extend Beachwatch to the west

People escape the heat at Lake Parramatta. ( ABC News: Lily Mayers )

But a spokesperson for the Office of Environment and Heritage said it had "no plans" to widen its Beachwatch program "at the moment".

Dr Khan said consistent testing at popular swimming sites along the west's riverways, were likely to reveal some bad results.

"There's a good chance, I think, that we'll find the rivers are not up to scratch, they're not ideally safe for swimming in at this time," he said.

Dr Wright said he would not suggest submerging eyes, ears or noses in water along the Hawkesbury River.

"It looks unclean to me. It has a little bit of a rich odour," he said.

"I would swim in it if I was really hot, but I would be mindful of the risks."