As the longwall mined through the fault productivity was reduced and out-of-seam dilution was increased.

Due to these geological conditions, Narrabri's FY21 managed RoM production guidance has been revised down to 5.4 - 6.0Mt from the previously announced 6.0 - 6.7Mt.

For the December quarter, managed RoM production at Narrabri of 1.0Mt was above the previous corresponding period however, the previous period included the effect of the longwall change out from LW108 to LW109.

"The sales mix for the quarter included a higher proportion of other thermal coal sales, of 0.4Mt, as a result of the out-of seam dilution," the company said.

"Equity metallurgical sales of PCI [pulverised coal injection] were 0.1Mt for the quarter."

The next longwall move from panel 109 to 110 is scheduled for Q4 FY21.