Whitehaven Coal has lowered the production guidance for its Narrabri mine in NSW by about six per cent after encountering "adverse geotechnical conditions" at the site.
The company did not provide specific details but said the revised guidance was a result of production impacts associated with managing the adverse conditions.
The east coast miner now expects its lowest cost and most productive mine to produce between 7.5 to 7.8 million tonnes of coal in 2016/17, down from its previous estimate of 8.0 to 8.3 million tonnes.
It had produced 6.9 million tonnes of coal in 2015/16.
The company is in the midst of a face widening project at the site, as it looks to expand production there. It said on Monday those changes were on track, with production expected to start in the longwall early in the June quarter.
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Whitehaven insisted it is still on track to achieve overall full-year production guidance of 20 to 22 million tonnes despite the issues at Narrabri and the impact of earlier wet weather.
The miner also said sales of metallurgical coal - used as a steelmaking ingredient - jumped to 22 per cent of total sales in the December quarter, compared to its normal average of 16 per cent in the previous three months.
It has set a medium-term target of increasing the proportion of the higher margin met coal to a third of its output .
Whitehaven will detail its December quarter production report on January 16.
Whitehaven shares closed 5.0 cents, or 1.81 per cent, lower at $2.72.
