A Queensland island resort whose guests and staff have been struck down by gastroenteritis, after E coli bacteria was found in the drinking water, boasted having "the most pure drinking water in southeast Queensland".

Tangalooma Island Resort on Moreton Island also claimed to have had an "almost unlimited supply of clear fresh water".

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Last week, more than 50 guests and staff began experiencing gastro symptoms including vomiting and diarrhoea.

Tests are being done on an underground aquifer and guests at the resort are being provided with free bottled water until the issue is resolved.

On its website, the resort says it sources its water from the underground water table below the island which contains thousands of litres of fresh water.

Accommodation at Tangalooma Island Resort. File image. Credit: Supplied via AAP

"Rain water that has fallen on the island over the past 10,000 years has been stored and filtered by the sand on Moreton Island," the website reads.

"It is an extremely efficient way of filtering the rainwater and we tap into this and supply the resort with the most pure and naturally potable water possible."

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

Management is working with authorities to determine how the contamination occurred.

"Somewhere, bacteria must've entered into the water supply - whether the storage tanks or a breaking in the piping or supplies," Dr Kari Jarvinen, from the Metro South Public Health Unit, told reporters.

"That's still being investigated as we speak."