Council water authorities say chlorine treatment on a contaminated reservoir in the Port Douglas and Mossman region of far north Queensland has failed to kill the bacteria E. coli.

Cairns Regional Council staff treated three reservoirs in Port Douglas, north of Cairns, where low contamination levels have been found.

The bacteria has been found in a total of seven water tanks in Port Douglas, Mowbray, Rocky Point and Daintree Village.

The council's general manager of water and waste, Bruce Gardiner, says more chlorine may be used.

"The other two reservoirs, we did a low dose of chlorine on Saturday afternoon and unfortunately we're still waiting on test results to come back from those two reservoirs," he said.

"The test takes about 18 hours in the laboratory once you actually take the sample and get it back to Cairns in the laboratory, so we're still waiting on those two."

Division 10 councillor Julia Leu says she is concerned that using chlorine to eradicate harmful bacteria could undermine the pristine reputation of Port Douglas.

She says the council should isolate and flush out the reservoirs before resorting to chemicals.

"It does threaten our tourism industry, it markets itself as one of the few tropical areas in the world that has 100 per cent pure water and certainly coming a few weeks before the upcoming school holidays it's a great concern to all the tourism operators in the shire," she said.