Tests are being carried out at Queensland's Tangalooma Resort after bacteria in the water caused staff and guests to be struck down with gastro.

More than 50 people have been struck down with gastroenteritis at one of southeast Queensland's popular island resorts after E coli bacteria was found in the drinking water.

Managers at Tangalooma Resort on Moreton Island became aware of the issue last week when guests and staff began experiencing vomiting and diarrhoea.

Government officials suspect an underground aquifer on the island was contaminated.

"It is a private water supply that is maintained and looked after by the facility managers," Metro South Health public health physician Kari Jarvinen said.

"Bacteria must have entered into the water supply, whether through the storage tanks or a breakage.

"We are still investigating and getting to understand exactly how their system works and where the contamination may have come from."

The water supply can be flushed or treated over several days and then monitored to make sure it is safe to drink, Dr Jarvinen said.

He has warned of dehydration among holidaymakers who have become ill.

Guests at the resort are being provided with free bottled water until the issue is resolved.

The resort has also contacted 3000 guests who stayed there in recent weeks prior to the outbreak.

Management is working closely with Brisbane City Council and Queensland Health to determine how the contamination occurred.

"We're just testing to see where the source of it is, how did it get into the water supply because it's never happened before and that's what's got us a little bit stumped here," resort director David James told ABC radio on Tuesday.

Meanwhile, the water reservoir was treated on Saturday and the resort is awaiting the results.

"All we can wait for is the test results to come through and hopefully today or tomorrow we get some better news and it's cleared up," he said.