AGL Energy's $200 million upgrade of its Bayswater power station is a missed opportunity to curb the country's largest source of asthma-inducing sulphur dioxide, says the not-for-profit legal service Environmental Justice Australia.

AGL recently trumpeted investment in the Hunter Valley plant, the company's largest, which increased its capacity by 100 megawatts through efficiency improvements "without additional emissions” or burning more coal.

Trucking on: Bayswater power station in the Hunter will become more efficient - but not necessarily cleaner. Credit:Rob Homer

However, Bayswater emitted 63,470 tonnes of SO2 in 2015-16, more than any other power plant in Australia - or even the US, where stricter curbs apply, EJA said, citing Australia's National Pollutant Inventory and the US Air Markets Program Data.

"AGL is spending significant money to expand the capacity of Bayswater, prolong its life and increase company profits," said Nicola Rivers, EJA's director of advocacy and research.