“Liddell is licensed to emit toxic pollution at levels that would see other power stations subject to serious enforcement actions.”

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EJA said coal-fired power plants account for almost half of Australia's NOx emissions, exposure to which is known to increase asthma and allergies, and lead to eye, nose, throat and lung irritation.

The environmental group also managed to obtain under FOI – after a six-month battle – a NOx report prepared for AGL by consultants Aurecon. It identified available control measures that could cut such pollution by as much as 90 per cent.

These technologies include altering furnace combustion and the use of so-called "wet scrubbers" or selective catalytic reduction, that have been compulsory in the US for about two decades, EJA said.

“Our regulatory approach and our community’s expectation is that filthy old cars will either be cleaned up or taken off the road,” Dr Wheland said.

“[AGL] owe it to the community," he said "There are health benefits every time NOx emissions are reduced."

An AGL spokesman said Liddell "operates in compliance with licence conditions set by the NSW EPA", adding that its environment protection licence conditions for NOx emissions "were originally determined prior to AGL’s acquisition of the power station in September 2014".

For its part, the EPA agreed AGL "is meeting its operational requirements" and added that the company "is in the process of lowering its emissions through a pollution reduction program", according to a spokeswoman.

She said AGL had objected to the release of the information under FOI requests because the report contained information it considered to be commercial in confidence.

Fairfax Media also sought comment from Don Harwin, the NSW energy minister.

AGL said it was planning to replace Liddell's capacity after 2022 "with modern, cleaner more reliable electricity generation".

Dr Whelan said AGL could use some of the estimated $250 million in after-tax earnings it expects to make from Liddell in its final years to clean up the emissions.

"AGL has said, 'We've a rogue power station here, that we’re going to close it down in an orderly fashion'," Dr Whelan said. "We support that. We just don’t support extensive emissions of NOx in the meantime."

"They could be bringing down asthma rates in the next month if they put their mind to it," he said.