The Liddell coal-fired power station was granted a special exemption allowing it to pump out toxic nitrous oxides at almost twice the rate allowed similarly aged plants, documents obtained under freedom of information show.

The Hunter Valley generator owned by AGL – which on Monday rejected a takoever bid by Alinta Energy for the plant – was last year given permission by the NSW Environment Protection Authority to emit NOx concentrations of 1400 micrograms per cubic metre.

That tally compared with 800mg for similarly-aged power stations in the state and just 50-100mg best practice emissions in nations such as Japan, according to Environmental Justice Australia.

AGL's Liddell power station in the Hunter Valley is due to close in 2022. Credit:Janie Barrett

The exemption runs until 2022, the year AGL plans to shut the plant that will be more than half a century old by then.