Hydro Tasmania increased the dam's discharge to eight cubic metres of water a second in a bid to flush faecal matter from First Basin.

The fourfold increase in water flows took place from about 4pm to 10pm.

The measure came four days after the Basin was closed for swimming but there is no guarantee it will work.

``This release of water is expected to have positive impacts on the bacteria readings, but there are many factors at issue here,'' Launceston City Council's general manager Robert Dobrzynski said.

``Some of the underlying causes behind the rise in bacteria levels will still be an issue.''

The council and river experts say that to track the source of contamination would be too difficult given the number of passible factors and the size of the catchment.

Potential factors include hot weather, agricultural run-off, untreated sewage to a ``larger than usual group of ducks'', according to the council.

``For us as a council to investigate the entirety of the two catchments would be impracticable. Especially if the readings are a result of natural phenomena,'' Mr Dobrzynski said.

Swimming holes on the North Esk at St Leonards and Corra Linn remain closed and will not be altered by the water release.

Meander Valley Council has closed Hadspen Spring Reserve to swimmers but water testing further downstream on Trevallyn Lake has not found unsafe levels of enterococci bacteria which indicates the presence of faecal matter.

A state government spokesman said it was ``monitoring the situation''.

The Launceston City Council requested Hydro increase water flows after tests results yesterday found high levels of enterococci.

Water levels in the Cataract Gorge were expected to remain high for about 12 hours after the discharge.

``In coming days we would urge people to pay attention to the warning signs that the council has in place at the different swimming areas around Launceston,'' Mr Dobrzynski said.

``Our council officers will continue to take water samples from the recreational swimming areas around Launceston until the end of March.'

The Health Department was unable to say if anybody had presented to hospital with symptoms consistent to enterococci exposure.