A sample of the tank water supply used for drinking has tested positive for contamination, which prompted an instruction from the Health Department to flush the pipes and clean tanks.

The college's 250 students were sent home early yesterday and will not return until Tuesday to allow cleaning crews to complete their task.

Assistant principal Jeff Keith said yesterday Moyne Shire's environmental health officer also took water samples.

"Samples were taken from a number of different points. We'll flush all pipes and make sure water tanks are purified," he said.

"Students were sent home because we have to take steps to ensure the safety of the site." Mr Keith said several dozen students and staff had contracted gastro recently, but an increase in victims this week prompted a check of the water.

"On Wednesday as a precaution we took a sample of our water to Hamilton for testing," he said.

"We also, as a precaution, stopped students from drinking the water and immediately brought in bottled water.

"On Thursday afternoon Southern Scientific Water informed us preliminary testing showed some of the water had been contaminated.

"The Department of Health was consulted and a cleaning schedule sent to us.

"Moyne Shire's environmental health officer suggested the cleaning could not be carried out while the school was occupied and that it should begin as soon as possible.

"We'll definitely have all classes back by Tuesday.

''Year 11 exams start on Wednesday," Mr Keith said yesterday.

The college uses bore water for washing and toilets.