The Launceston City Council has also shut swimming holes at Corra Linn and St Leonards on the North Esk River due to the presence of the enterococcus bacteria.

The Meander Valley Council has closed Hadspen River Reserve on the South Esk River, upstream of the First Basin, because of the contamination.

Launceston will await test results on Friday before discussing the option of a water release with the Hydro.

The Northern Midlands Council said testing at Longford had not found the bacteria.

Enterococcus is an indicator organism used to monitor for the presence of other, more harmful organisms in recreational water.

It is present in the intestines of humans and other warm-blooded animals such as livestock and wildlife, and therefore is an indicator of faecal contamination.

The recent testing at the First Basin recorded 885 enterococci per 100 millilitres of water, more than six times the safe level.

Exposure to enterococci above 501 is known to cause respiratory and gastro-intestinal illness.

Curtin University Professor Warren Grubb said yesterday flushing would help improve water quality, but it was important to find the source of the contamination.

He said it was unusual to discover such high levels of enterococci in water.

Director of Public Health Roscoe Taylor said it was likely e-coli was also present in the rivers.

Discovering the source of the faecal matter was more complex, he said.

Animal waste from farms, overflowing septic tanks and sewerage system leaks were all potential causes.

Ben Lomond Water operates several sewerage plants upstream of the contaminated sites on the South Esk but not the North Esk.

Ben Lomond chief executive Barry Cash said this indicated the sewerage plants could not be responsible but confirmed they regularly failed to meet licence requirements.

Launceston City Council general manager Robert Dobrzynski said it was too complex for the council to track down the source of bacteria.