An outbreak of a diarrhoeal disease associated with swimming has parents in three states being warned to keep kids out of pools and water parks when ill.

More than 1500 cases of cryptosporidiosis have been diagnosed in Queensland, NSW and Victoria in the past six weeks.

That's between five and nine times the recent average for the same time in each state.

More than half of cases have been among children aged under 10 and adults in their 30s.

As it approached 500 cases for the year, NSW urged the community to stay out of swimming spots for at least two weeks after they've had diarrhoea.

"Almost half of the cases reported this year had been swimming during their exposure period, and with such a high proportion of young children affected and with many schools about to hold swimming carnivals, we're urging parents to stay alert for symptoms," Health Protection NSW executive Jeremy McAnulty said on Tuesday.

Authorities in Queensland last week warned the disease could also be spread through contact with infected individuals or animals.

The 736 cases there in January, mostly centred on Brisbane, surpassed the yearly totals for 2021 and 2022.

Most of Victoria's 266 cases this year are children or those aged between 25 and 44.

Greater Geelong and the inner Melbourne council areas of Merri-bek and Yarra have recorded the most cases.

The disease is caused by a parasite about the width of spider silk that infects the intestine, commonly causing acute diarrhoea in young children.

It can also cause nausea, vomiting, fever, headache, and loss of appetite.

Drinking or accidentally swallowing water is a common method of transmission but the parasite can survive for many days outside the body, even in chlorinated pools.

There is no specific treatment for cryptosporidiosis but it is important to stay hydrated, Queensland Health says.

Once symptoms resolve, people who have had diarrhoea should not handle food for 48 hours, swim for at least two weeks or share towels for at least two weeks.

Children who have diarrhoea should be kept home from preschool, childcare or playgroup until 24 hours after the diarrhoea has completely stopped, NSW Health said.

People can reduce the risk of catching cryptosporidiosis by not swimming at ocean beaches within a day of heavy rain and in estuaries and inland waterways within three days of heavy rain.

Drinking untreated water that has not been boiled and failing to wash hands after handling animals or changing nappies increases the risk of consuming the parasite.