Hunter Valley mines have been issued five penalty notices and two official cautions over the past two years despite dozens of instances where air quality guidelines appear to have been exceeded.
A report commissioned by Lock the Gate highlighted multiple cases where four mines - Mount Pleasant and Bengalla and Muswellbrook coal and Mt Arthur - had exceeded air quality criteria but had not been penalised.
In its response to the report, the Department of Planning said air quality impact conditions included the requirement to implement "reasonable and feasible mitigation measures" to avoid exceedances of air quality criteria.
"Accordingly, in cases where measured total results (including background) exceed the criteria, operations may not necessarily be in non-compliance provided they can demonstrate that reasonable and feasible mitigation measures were implemented," Acting deputy secretary, Planning and Assessment David Gainsford said.
Mitigation measures can include changing operational work faces and dumping locations during adverse weather conditions, the use of water to suppress dust emissions from haul roads and coal stockpiles or ceasing mining operations at relevant times.
Mines are required to immediately notify the department of air quality exceedances - a practice which environment groups have widely criticised.
The department rejected an assertion from Lock the Gate that mines had been allowed to breach their consent conditions through the incremental application of air quality standards.
Mr Gainsford said where a mine's contribution exceeded criteria, a notification was required to be provided in accordance with their conditions of consent," he said.
Mount Pleasant and Bengalla and Muswellbrook coal mines did not comment about the situation.
A spokesman for the BHP-owned Mt Arthur mine said the company appreciated thatdust was an issue for surrounding communities and it continued to work with industry, government and the community to manage air quality.
"BHP actively monitors the impacts of our operations with a real time dust monitoring system at Mt Arthur. It enables mine activity to be managed efficiently while preventing dust levels from exceeding regulatory requirements," he said.
"We report on any events or exceedance and have a range of measures to control dust and improve air quality."
A Department of Planning Industry and Environment spokeswoman said Hunter Valley mines were required to publicly report exceedances of air quality conditions.
"The department enforces strict compliance with consent conditions and any breach may result in a fine to the company," she said.
Lock the Gate Spokeswoman Georgina Woods said the department's enforcement of air quality exceedances at Hunter mines was of little comfort for people who lived with poor air quality.
"If application of reasonable and feasible measures is not enough to achieve the national air quality standard, that seems to be an argument that there is simply too much exposed land among the large-scale open cuts near to population centres and a critical threshold has been reached," she said.
She said the department's methodology amounted to an "accounting trick", which allowed air pollution to increase across the Hunter Valley.
"Planning Minister Rob Stokes must launch an investigation into air pollution in the Hunter and the systematic evasion of environmental standards by coal mining companies with the complicity of his own department," she said.
