This is the warning from Dr Ian Wright, senior lecturer in water science at Western Sydney University (WSU) Hawkesbury campus, who said it "makes me cringe" seeing footage of people playing in flood waters.

It upsets him even more that people faced with flood clean-up might not know the dangers that lurk in the muck and could be putting their health at risk by failing to take the necessary precautions to protect themselves against bacteria that could cause serious infections.

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"Flood water looks and smells contaminated, because it is. Your senses are warning you of danger," Dr Wright told the Hawkesbury Gazette.

After the March 2021 flood in the Hawkesbury Valley, Dr Wright led his students on an expedition to test flood waters in the Hawkesbury River at Windsor, Rickabys Creek at South Windsor and the Hawkesbury-Nepean River at Yarramundi.

He said the nutrient levels were elevated, it was very turbid - a cloudy, muddy colour - but the thing to worry about most was the bacterial count, in particular faecal bacteria, which was "sky high".

"People should avoid any contact with flood waters," Dr Wright said.

"The poor people that have to deal with flood clean-up should limit contact, wear gloves, even masks. If people have poor immunity, are very young or elderly, they need to be careful."

He said he doesn't think people "really understand how infectious flood waters are".

"I would recommend people treat flood water with the utmost respect, and also the residue that's left. It's well-known that with the urban water run-off into creeks, estuaries and beaches when you've had heavy rain, basically the sewage system has been over-flowed," he said.

"All the faecal waste from humans, farm animals and domestic pets has suddenly been mobilised. It's diluted sewage.

"Contact with flood water could be the thing that puts a vulnerable person in hospital with a raging infection."

Dr Wright also warned people in flood-affected areas should wear mosquito repellent to ward off mosquito-borne diseases including Japanese encephalitis, Ross River fever and Barmah Forest virus.

"Wetter and warmer environments are good for the spread of these diseases and the mosquitoes spreading them," Dr Wright said.