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.

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.

"It is believed that emission limits may be exceeded when the sulphur content reaches 0.8 per cent," the note said. Costain at 0.38-0.51 per cent was the lowest.

Two decades on in 2010, the allowable pollution rate has been cut for sulphur, with another note advising staff: "Consistent and conscientious efforts must be made to ensure that only coal with a calculated emission level of 535 ppm [sulphur dioxide] or less is delivered to the Unit bunkers. This level should only be exceeded with the approval of management."

The note also provides staff with a handy diagram explaining how to manage high-sulphur coal:

A former Bayswater employee said the rules on blending coal only applied to Unit One. The other three "weren't being monitored", he said.

AGL bought Bayswater from state-owned Macquarie Generation in 2014. The company told Fairfax last week that all four units were now being monitored. Fairfax sought further comment and whether the process revealed at Bayswater applied to the Liddell station that AGL also acquired from MacGen.

"Liddell and Bayswater power stations operate in compliance with licence conditions set by the NSW EPA, Protection of the Environment (Clean Air) Regulation 2010 and national air quality guidelines," an AGL spokesman said, declining to comment on operations prior to the 2014 takeovers.

A spokesman for Environment Minister Gabrielle Upton reiterated comments that "the EPA is currently looking at the allegations. We will respond as appropriate".

"The EPA will consider this as part of our review of power station monitoring," an EPA spokeswoman said, referring to the documents.

NSW Greens energy spokesperson Jeremy Buckingham called for "a thorough investigation into how widespread this practice has been and how long it has been happening".

"It's clear that this deceit has been occurring for a long time and it's improbable that no one at the EPA or in government knew," Mr Buckingham said.

"These are substances that can be highly damaging to human health and the environment, so any manipulation to avoid the pollution limits is a very serious issue."

Adam Searle, Labor's energy spokesman, said the documents add to concern that Bayswater and other power plants may have sought to mask emissions and saved on pollution fees as a result.

"It opens a whole host of issues about compliance now and over the past couple of decades," Mr Searle said. "But it's also about what these power stations are doing today."