Environmental Justice Australia (EJA) audited almost three months of data from the four stations operated by Whitehaven and Idemitsu, and found 55 per cent of the data showed readings below zero.

To put that in perspective, EJA researcher James Whelan said the EPA ran a state-wide network of 45 air monitoring stations, none of which have recorded PM10 (particle levels) readings below five.

“You can’t physically get below zero, not even in the most remote parts of the state, let alone in the middle of five big coal mines,” Mr Whelan said.

“We aren’t accusing the companies of deliberately doing it, but it shows the system is clearly not working.”

The Environmental Protection Agency (EPA) commissioned an audit of the Maules Creek monitoring station earlier this year, in response to another EJA review which raised concerns about air quality in the region.

The EPA will consider if any regulatory action is required once the audit’s findings are reviewed.

“Negative values can be within the normal range for the type of monitoring instruments used at the Maules Creek station, for instance when particle levels are very low or as a result of sudden changes in temperature and humidity,” an EPA spokesperson said.

An Idemitsu spokesperson said it took its compliance requirements very seriously and continued to meet all obligations for dust monitoring. Whitehaven was approached by The Leader but declined to comment.

Wando Conservation and Cultural Centre spokeswoman Pat Schultz said reporting negative pollution levels added “insult to injury”.

“These coal mines expose our communities to dangerous pollution, the least they can do is to report accurately,” Ms Schultz said.

A maximum penalty of $1m can apply to mining companies that submit false or misleading information to the state government.