Oodgeroo MP Mark Robinson has called for gates to be reinstalled at Leslie Harrison Dam to stop drinking water escaping down the spillway and into Tingalpa Creek.
Mr Robinson said downpours that had brought more than 250 millimetres of rain to the Redlands and caused the dam to spill could have been stored and used to drought-proof the region.
He said it was unacceptable that money had not been allocated to restore the flood gates after they were taken down in 2014 for maintenance work.
He rubbished claims that the south-east Queensland water grid would provide drought-proofing, arguing that the dam was doing the job before the gates were removed.
"If the gates had been put back on we would probably have a full dam," Mr Robinson said.
"I feel really angry and frustrated about it as a local myself and on behalf of other locals because it is a pure waste, especially in light of the drought declaration over the Redlands. We are not immune from drought.
"We have had about six years of Seqwater downgrading our dam and therefore saying they don't have to spend the funds but that is not acceptable and the community doesn't find that acceptable.
"Any petitions or community events that have happened in the last few years, people have turned up in their droves because they don't accept having half a dam."
An Seqwater spokeswoman said they had considered reinstalling the gates as part of its upgrade but found it would cause a flood risk to people downstream, as the dam's water levels would rise above the probable maximum flood height in the event of extreme weather.
"Removing the gates returned the dam to its original design capacity," she said.
"Lowering water levels is an industry-accepted practice to best manage the safety of dams either temporarily or long term.
"To return the previous water level we would need to raise the height of the dam wall by three metres and acquire neighbouring properties at a cost of over $18 million.
"When the gates were first installed, Leslie Harrison Dam was the sole water supply for Redlands. Since that time Redlands has been connected to North Stradbroke Island and the south-east Queensland water grid, providing Redlands with one of our region's most secure water supplies."
Mr Robinson said rates spent on the dam when it was under council control had gone to waste.
"The state paid very little for the dam itself. We had very cheap water in the Redlands and now it is very expensive," he said.
"The water grid has not drought-proofed the Redlands because it already was drought-proofed. It had a fully functional dam.
"...Now the risk is we have got half a dam connected to a grid. I would rather have our whole dam full of water and I would rather look after our own local supply."
The Seqwater spokeswoman said the dam had spilled nines times since the gates were taken down.
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