NSW Health is offering to help water suppliers test for PFAS "forever chemicals" following a growing body of evidence suggesting the cancer-linked substances are far more widespread across the state's water catchments than previously thought.

The health authority has told the ABC it will assist local water utilities to screen for the manmade substances.

Councils across NSW are responsible for supplying drinking water to more than 1.8 million people outside of the Sydney and Hunter regions.

It comes as water scientists have warned a "knowledge gap" and lack of accountability over the scale and nature of PFAS across NSW may be putting people at risk.

"If this was a jigsaw puzzle, we only have about 5 per cent of the pieces on the table," Western Sydney University water scientist associate professor Ian Wright said.

The compound — known for its heat, stain, grease and water-repellent qualities — is found in everyday products such as food packaging and clothing.

PFAS moves through water and does not break down in the environment.

The US Environmental Protection Agency considers there is no safe level of PFAS in drinking water, due to health risks it presents to humans, but the Australian guidelines state there is a safe level of exposure.

Australia's drinking water guidelines are currently under review.

It has only been through ad hoc testing by scientists that new hotspots in NSW are being discovered, outside of the originally established Defence Force bases and airports where contaminated firefighting foam was widely used.

"When we sample, we find it," Dr Wright said.

"Our water authorities, our environment authorities, our health authorities — why aren't we sampling everywhere and sharing this information?" he said.

'Massive knowledge gap' in PFAS sources

In June, Sydney Water increased its testing for PFAS across the city's drinking water catchment.

At the time, it said, the additional sites were tested because of media pressure.

As a result, it detected PFAS at several water filtration plants, with the highest levels at Blackheath and Katoomba in the Blue Mountains — but reiterated they were safe under Australia's current drinking water guidelines.

Prior to that, Sydney Water was only regularly monitoring one site in the catchment, North Richmond, near where PFAS-contaminated firefighting foam was used at Richmond RAAF base.

Additional tests by Dr Wright in early September, revealed by the Sydney Morning Herald on Tuesday, found a Blue Mountains dam in Sydney's drinking water catchment had PFAS levels at more than 50 times the Australian drinking water standards.

WaterNSW said it was conducting its own testing of water sources in the Blue Mountains catchment to investigate the source and causes of the elevated PFAS results.

It has also shut off a pipeline from Medlow Dam and Greaves Creek Dam that supplies almost 50,000 people "as a precaution".

Last month, the ABC revealed an Australian-first study by the University of Western Sydney found high levels of PFOS, a group of PFAS chemicals, in platypuses.

That study, again independent of government, showed the substance had bioaccumulated in the native species, even in remote areas such as the Tumut River east of Wagga Wagga.

Another story by the ABC raised the alarm about a stretch of the Belubula River, in the state's Central West, being covered in foam containing PFOS.

The NSW EPA is now conducting testing there.

"There is a massive knowledge gap … in most cases, we have no idea where it [PFAS] is coming from," Dr Wright said.

He is particularly concerned regional water users will be the biggest sufferers.

"This is a wake-up call for the water industry right across the state."

Calls to broaden the definition of PFAS hotspots

Water utilities are responsible for complying with the Australian Drinking Water Guidelines by controlling sources of contamination in their catchments.

NSW Health is responsible for ensuring the water utilities are meeting those regulations.

The health authority "recommends" that water utilities assess the risk of PFAS contamination in their catchment.

Whether that has been occurring is another question.

It is now recommending they undertake screening for PFAS "regardless of the outcome of the risk assessment".

NSW Health will arrange for testing of one sample of treated drinking water from each supply system that has not yet had any PFAS testing and provide an initial screening result.

It will also arrange for a sampling kit to be provided and samples to be sent to an approved laboratory.

University of Sydney's head of civil engineering Professor Stuart Khan said authorities needed to be on the front foot and start identifying sites of concern themselves.

He said we needed to broaden the definition of PFAS hotspots to include waterways around highways, where firefighting foam may have been used for a car accident, and places of light industrial activity.

"The risk might be that we're retrospectively discovering that there have been sites where people might have been exposed to unsafe levels over a long period of time," Dr Khan said.

But improving water quality monitoring, which Dr Khan said had broadly been recognised as an area Australia had dropped the ball, would be a costly exercise.

"It will require resourcing, but I think there's probably a very clear social licence for that in Australia right now," Dr Khan said.

In a statement, a NSW government spokesperson said "out of an abundance of caution" Sydney Water and WaterNSW had added PFAS monitoring to their "rigorous testing procedures".

"Testing protocols are currently being assessed, and we are open to increasing the frequency and geographical scope of testing, should it be deemed necessary."

It said should any resourcing concerns arise, NSW Water Minister Rose Jackson was "prepared to address them swiftly to ensure continued water safety".