BHP Billiton's diamond production totaled 457,000 carats in the September quarter, down 35% year-over-year and 21% compared to the June ending quarter.

Production was lower than all comparable periods due to the ongoing decline in average ore grades and unseasonably high rainfall that affected operations, the mining giant said in a quarterly production report.

Production at Ekati, the company's sole diamond mine, is expected to be impacted in the medium term as operations transition to lower grade material, consistent with the mine plan, it added.