Sydney Water is being accused of covering up the real cause of a water contamination that led to dozens of residents in Sydney's south east to fall ill.
In late December, more than 100 residents around Botany complained after drinking water that many described as smelling like gas and tasting of disinfectant.
On the day of the contamination there was a leak of toxic groundwater at the nearby Orica chemical plant.
Sydney Water told residents that the problem was caused by bitumen getting into water pipes during maintenance.
Independent experts have questioned the water authority's explanation.
"If this wasn't deliberate it was plain incompetent. And that's absolutely not good enough.
Ms Faehrmann's pointed to the chemical leak at the Orica plant.
"A broken pipe or leak may have contaminated Sydney water", she said.
Affected residents have received a rebate from Sydney Water as compensation.
Premier Barry O'Farrell dismissed claims of a cover-up.
"The fact is the Department of Health is the independent water regulator and they are very clear that the levels of chemicals in the water supply were within the national health and medical research centre's standards," he said.
"And they are very clear that it did not pose a threat to health."