The approval to mine six longwalls at Wongawilli Colliery was granted in 2011 and expires in December this year. But, in an application to NSW Planning which went on exhibition this week, the firm's group environment manager David Clarkson said only a fraction of the approved mining work had been completed.

"Only 8.5 months of development work and longwall coal extraction has been undertaken to date, and longwall extraction has only occurred within one of the six approved longwall panels," he said. "[Wollongong Coal] has extracted only 497,240 tonnes of the 3.3 million tonne coal resource in the Nebo Area.

"The remainder of the approved operations cannot be completed within the approved duration of mining operations."

Despite the November 2011 approval, mining at the firm's southern Illawarra colliery - which was then owned by Gujarat NRE Coking Coal - did not begin until June 2013 and stopped abruptly eight months later. In March 2014, a roof collapse at the mine buried a costly piece of longwall mining equipment, and hampered the extraction of coal.

Then in May that year, more than 100 workers at the mine were made redundant, after Construction, Forestry, Mining and Energy Union members unanimously rejected what they said was a "ridiculous, unrealistic" pay offer, which could have meant some workers lost 40 per cent of their pay. Since then, the mine has been running on "care and maintenance", with no mining or production work.

Mr Clarkson said extending the approval until December 2020 would allow the employment of "up to 300 personnel", and would not change the environmental effects of the mine.

"Given that the modification does not involve the enlargement or intensification of mining operations at Wongawilli Colliery, the impacts of the modified project will be the same as the impacts of the approved project . . . [so] no additional mitigation measures are considered necessary," he said.

"By allowing the approved mining activities to be undertaken, the modification will allow the full economic benefits of the project to be realised."

Meantime, Wollongong Coal has been suspended from trading on the Australian Securities Exchange since June 1 for breaching its reporting obligations.

It remains suspended for failing to lodge a report in line with the ASX's listing rules.

The planning department is seeking the community's views on the application to extend mining at Wongawilli, with submissions open until July 6.

Details of the plan and how to make a submission are available at majorprojects.planning.nsw.gov.au.