Managers of Bayswater power station in the Hunter Valley provided staff with precise instructions on how to blend coal to avoid triggering pollution alerts, documents stretching across almost two decades show.

As Fairfax Media reported, the NSW Environment Protection Authority has contacted all coal-fired power stations to determine whether operators took advantage of partial monitoring to conceal the full extent of their emissions of sulphur, nitrous oxides and other pollutants.

Bayswater power station in the Hunter Valley is being investigated after reports its blended coal to hide its emissions. Credit:Glen McCurtayne

Plant operating notes obtained by Fairfax show that as far back as 1991 Bayswater staff were told they had to keep sulphur emissions below 600 parts per million for Unit One because "any exceedence of this limit is reportable to the State Pollution Control Commission". The other three units' emissions were not reportable.

That 1991 note also outlines the range of sulphur content in the coal sourced from different mines in the Hunter, with Drayton, Bayswater, Muswellbrook, Novacoal and Liddell State all between 0.6-1 per cent.