A FIVE-year-old boy has been hospitalised for cryptosporidiosis after claims the water he drank from a garden tap was contaminated.

Joshua Douglas-Smith became violently ill with stomach cramps and gastric seven days ago, when he drank from an outdoor tap at his Lethbridge Park home in Sydney's west.

Test results by Nepean Hospital confirm Joshua had the highly infectious disease.

But the State Government yesterday denied Sydney's drinking water was contaminated.

Despite its reassurances, Joshua's parents Donna Douglas and Michael Smith remain unconvinced.

"He used to weigh 20kg, he now weighs 16kg," Ms Douglas said.

"Doctors told me it must have come from the water. I filled the cat bowl with water from the same tap and it was cloudy. It even smelt funny.

"I knew something wasn't right."

Cryptosporidiosis is a diarrhoeal disease caused by a parasite infection in the intestine. It can cause vomiting, fever, cramps and diarrhoea.

It is mainly transmitted through drinking untreated water, swimming and hand-to-mouth contact.

A sharp increase of 129 cases reported in November prompted health authorities to issue a warning. The number dropped to 100 in December.

NSW Health attributed the rise to swimming pools, which are a haven for spreading the disease.

Joshua's father said his son has not been able to drink or eat properly since last weekend.

Joshua was initially taken to Mt Druitt Hospital, where he was diagnosed with a gastro bug and sent home.

He was then seen by his GP, who carried out tests confirming cryptosporidiosis and transferred him to Nepean Hospital.

******   Lost 5kg ... Joshua Douglas-Smith  ******

"He is nothing but skin and bones now and he has no energy. He's lost more than five kilos," Mr Smith said.

"He has got nothing left in him and (was) on the drip to get back some of his fluids.

"He is usually full of energy and he's just been lying there and doesn't want to eat anything and all he wants to do is sleep."

Doctors released Joshua yesterday afternoon but it will be at least three more days before he fully recovers.

Sydney Water conducts daily testing of the drinking water supply, including checks for cryptosporidium and giardia.

A spokesman said there was nothing abnormal in the results to indicate the water was contaminated.