Many people know not to go swimming in the ocean after heavy rainfall, but a paddle after an unprecedented flood may expose you to serious biohazards.

NSW Health Minister Brad Hazzard said swimming after a one-in-100 year flood may mean you're paddling in diarrhoea.

"We know when we've had big rains, the general advice is don't go surfing, don't go swimming in the open ocean for a day or two," Dr Hazzard said.

"When you've had a one-in-100 year flood, you might be wise to not go surfing with parasites, bacteria and diarrhoea."

Mr Hazzard said flooding and sewage outflows posed a risk of cryptosporidium, giardia and other parasites and bacteria.

"Normally you've got to fight somebody away from the surf - this time you might well be fighting off the parasites for diarrhoea," he said.

Acting Chief Health Officer from NSW Health Dr Marianne Gale said after floods there were a number of risks posed to health by contaminated water.

She said people can look to advice from local councils around the safety of the water and relevant water authorities.

"After floods, many people may have had cuts and abrasions, and it's really important to treat wounds very carefully as well," Dr Gale said.

She urged the community to follow advice from other authorities regarding swimming in safe water.

People should make sure their drinking water is safe and properly treated and pay attention to hand hygiene, she added.