It follows a complaint made by a coalition of environment groups that alleged Delta had failed to comply with licence conditions.

Delta has rejected the claims and welcomed the investigation.

The coalition includes Environmental Justice Australia, Nature Conservation Council NSW, Lake Macquarie & Central Coast Coal Ash Alliance, Keep Lake Macquarie Clean, Greenpeace, Australian Conservation Foundation, Doctors for the Environment, Lung Foundation Australia, Asthma Australia and Healthy Futures.

The complaint refers to recently released National Pollutant Inventory data that showed Vales Point increased emissions of coarse particulates by 121 per cent and fine particulates by 181 per cent in 2018/19 compared to the previous year.

Company spokesman Steve Gurney accused the coalition of attempting to shut down coal-fired power stations.

"Once again we see activist groups seeking to create community fear based on selective and misrepresented data," he said.

"Delta questions why these activist groups choose to disregard the 2016/17 year and the fact that the emissions dropped in 2017/18, possibly because it doesn't support their sensationalist agenda."

He said results 'bounce around' from year to year because of the annualisation of three to four days of testing data.

"If you then take this 3-4 days of data and annualise it, you accentuate what was a small difference. As part of a review of Delta's Environmental Licence, this type of monitoring is being moved to a quarterly basis (rather than annual) which should result in more consistent data."

He said power station's 2018/19 stack emissions fell within the operating range for the plant and were well within compliance limits.

Environmental Justice Australia principal lawyer Nick Witherow said Delta's attempts to explain it haven't been properly controlling dangerous particle pollution defied logic.

"It appears that Delta Electricity has breached the law and its licence conditions by failing to operate and/ or maintain plant and equipment at Vales Point in a proper and efficient manner, which has caused air pollution and put the community at risk. The EPA must investigate and hold them accountable," he said.

"We are aware that the EPA does not usually refer to the National Pollutant Inventory data in relation to a polluter's compliance with their licence and the Environment Protection Act. However, the steep and unexplained rise in overall particulate emissions Vales Point reported to the NPI warrants investigation. It also raises questions about the current means of reporting used to measure compliance where a power station is left to self-report any exceedances of stack emissions at any given time, especially if it has not picked up or acted on such enormous increases in toxic pollution."

An EPA spokeswoman said the authority would take decisive action against any licensee which was not complying with their licence.

"The EPA has a comprehensive and robust framework for regulating power stations in NSW and will take decisive action in response to any licensee who does not adhere to the strict requirements in place to safeguard the community and the environment," she said.

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