At this stage the cause of the incident has not been determined, which means the company cannot pursue development at one of its leading assets and obtain optimum value from its longwall top coal caving system.

"Yancoal confirms that development has not resumed at the Austar coal mine since the incident on 16 April 2014," it said.

"Train loading and washing of stockpile coal continues and relocation of the longwall and associated equipment from longwall panel A7 is underway."

In line with existing operational schedules the longwall will be relocated to panel A8 over the coming 8-10 weeks and is expected to provide up to 12 months of production.

The investigation into the incident by the Mine Safety Office of the NSW Department of Trade and Investment, Regional Infrastructure and Services is ongoing and Yancoal is working with DTIRIS.

"The incident site remains under the control of DTIRIS," Yancoal said.

"DTIRIS has not given Yancoal any advice as to when the preliminary investigation is likely to be concluded and access to the development area will be granted."

The mine's risks also include the timely delivery of the upgrades to the belt conveyor system, the successful relocation of the longwall from LWA7 to LWA8 and underground mining conditions.

The mine allocated $42 million in 2013 for capital expenditure, including $10 million associated with the stage 3 project, which underpins the future of the operation.

The 2014 operations focus was on ramping up the stage 3 project and delivering 1.9-2.1 million tonnes of production by targeting the Greta seam.

The Greta seam is 400-600m below the surface.

"This is the result of more than four years work to bring this new mining area into production and marks a significant milestone for the Austar mine," Yancoal said in its annual report.

"Significant planning and investment, including upgraded infrastructure and logistics, have been made to deconstrain the operation and help achieve more consistent production and minimise inherent risks."