Around 4000 Brisbane residents affected by a surge of fluoride in their water will receive a written apology, as debate raged over the quality of the city’s water.

Premier Anna Bligh yesterday revealed 300,000 litres of contaminated water was pumped to northern Brisbane homes for three hours on May 1 after a plant malfunction delivered 20 times the allowable limit of fluoride into the water supply.

The fault was uncovered during routine tests, but health authorities said it did not pose a risk.

Ms Bligh said health and environmental authorities would be writing to affected residents in the Warner and Brendale areas.

“I think it should contain factual and accurate information about what happened and, yes, some form of apology about this happening,” she told ABC radio.

“It’s not acceptable. This is something Queenslanders should be able to rely on and in this case they haven’t been able to.”

Ms Bligh said an independent investigation now under way could have broader implications for the water system.

“Maybe there are some long terms lessons for us here, not only in relation to fluoride but other water quality issues,” she said.

“I drink the water here and I understand the people want to have a very high level of confidence in their water supply and in the systems that manage its purity.”

Opposition frontbencher Tim Nicholls said the fault raised further questions about the safety of other parts of the state’s water system.

Mr Nicholls said it was particularly important to get the problems solved, given the government’s controversial commitment to introduce recycled water if the state falls back into drought.

“There should be fail-safes along the way, particularly with recycled water where there is real prospect of harm from that water coming through if something has gone wrong, if it hasn’t been properly treated,” Mr Nicholls said.

“It calls into question the government and its operator’s capacity to be able to monitor the water and to reassure the residents that they are getting safe drinking water.”

Injury compensation law expert Mark O’Connor said laws introducing fluoride to Queensland’s water supply contained a clause banning legal action for compensation if problems should arise.

Mr O’Connor said despite the premier and Queensland Health reassuring the public the risk of illness was extremely remote, the government should offer to pay for any medical tests householders might wish to take.

“People are expected to put their trust in the government and when the government fails them, it has a duty to put things right,” Mr O’Connor said.

“Paying for any medical tests is the least it could do.”