Production at South32's Illawarra Metallurgical Coal sites fell by a third in the past three months, as the company began a longwall move.
During that time the company was also hit by six weeks of strike actions.
The Perth-based miner which owns and operates the Appin and Dendrobium mines in the Illawarra saw total coal production fall by 33 per cent or 585 kilotonnes in the September 2023 quarter.
At Dendrobium, the company began an extended planned longwall move which contributed to the drop in production.
A longwall move is where mining equipment moves along the longwall or seam to access a new section of the material.
Additional longwall moves are expected later in the financial year, with two in the current December quarter and two in the June quarter next year.
The company expects to produce 5,000 kilotonnes of coal over the 2024 financial year.
South32's Illawarra operations were also hit by six weeks of strike actions as staff from the Collieries Staff and Officials Association walked off the job in August.
Roughly 80 workers at the Appin mine represented by the CSOA and the Mining and Energy Union took industrial action during negotiations over pay and conditions. At issue was access to time off on public holidays and annual leave during holiday periods such as Christmas.
While these staff were on strike, South32 stood down other workers at Appin due to safety requirements, but at the time CEO Graham Kerr told investors production was not materially impacted by industrial action.
South32 invested US$180m in productivity and improvement activities, which included the transition to a single longwall configuration at Appin from the 2025 financial year and additional ventilation capacity. This will enable mining in Appin until at least 2039.
Last year, the company announced it would not expand the Dendrobium mine in Mount Kembla which would have extended operations until 2041. Under current approvals, South32 can continue mining at Dendrobium until at least 2028, with options to extend to 2032.
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