Wellington health authorities are working to contain an outbreak of the illness cryptosporidium and are warning people with gastrointestinal symptoms to stay out of public pools.
Cryptosporidium is a parasite that causes symptoms such as stomach cramps and large amounts of watery diarrhoea.
Most people recover within a couple of days, but some can be ill for up to a fortnight, and those with compromised immune systems can become seriously ill.
About half the cases recently reported to Regional Public Health are those of people who have used public swimming pools around the region, in particular the Wellington Regional Aquatic Centre at Kilbirnie and the Arena Aquatic Centre at Porirua.
Regional Public Health says it is working with pool managers to ensure good systems are in place to reduce the spread of the illness.
People with diarrhoea are asked to stay away from pools for a fortnight after the onset of symptoms.