BHP Group reported a 5% decline in copper production for the quarter ending June 30.
Output fell to 491,900 tonnes during the period.
The results missed analyst expectations.
Lower production at the Escondida and Pampa Norte operations in Chile drove the decline.
The world's largest miner warned of further production decreases for the upcoming fiscal year.
BHP forecasts annual output between 1.65 million and 1.80 million tonnes.
This target represents a decline from the 1.95 million tonnes produced in fiscal 2026.
Lower ore quality at Escondida, the world's largest copper mine, causes the expected shortfall.