BHP Billiton Group slashed its fiscal 2019 copper production forecast to between 1.62 million and 1.71 million tonnes, from an earlier estimate of between 1.68 million and 1.77 million tonnes.

The revision comes on the back of lower output volumes at the company's Olympic Dam mine in South Australia and the Spence mine in Chile due to unplanned outages.

Olympic Dam is now expected to produce between 170,000 and 180,000 tonnes of copper as compared to a previous forecast of 200,000 to 220,000 tonnes, while the outlook for Spence was lowered to 160,000 to 175,000 tonnes from an earlier range of 185,000 to 200,000 tonnes.

BHP recently said it was targeting restarting an acid plant at Olympic Dam in October following repairs, after the company reported an outage at the plant in August.

In September, a fire forced the company to shut down operations at Spence. The mine is expected to return to full capacity in the December quarter.

In the first quarter, the company's copper output was broadly flat at 409,000 tonnes, while zinc production rose 5% to around 30,558 tonnes.

Iron ore output in the quarter rose 10% year over year to 61.39 million tonnes, as operations in Western Australia saw increased volumes, supported by record production at Jimblebar and improved reliability across the company's rail network and port operations.

Metallurgical and energy coal output dropped 2% and 1% to 10.36 million tonnes and 6.64 million tonnes, respectively.

The company's nickel output from its Nickel West operations in Western Australia also fell 8% year over year to 21,400 tonnes.

Operations at the Kalgoorlie smelter were suspended following a fire that damaged a localized area in the furnace building. The smelter returned to operation Oct. 1 and is expected to ramp up to full capacity from early November.

BHP maintained its full-year production forecast for all other commodities.