Rio Tinto has officially opened its underground operation at the Argyle diamond mine in the Kimberley, following the exhaustion of the site's open-cut operation.
The company said it started underground production on the first of April, but peak production would not be achieved until around 2015.
The move underground is expected to extend the mine's life to at least 2020, with the open pit operation finishing up this year.
Rio has made a number of reductions to the Argyle site over the last 12 months, and last year announced plans to sell off its diamond assets, including Argyle.
The company said in its new phase Argyle was expected to produce 20 million carats a year, and the site was still up for sale.