Previously, the company projected to start production at the facility by the end of the first half of 2016.
The company said it was able to get ahead on various work streams across the project before the start of the 2014/2015 summer rainy season.
However, the works were affected by the impact of an increase in the quantity of overburden to be eliminated from the main crusher and plant terraces.
The grid power project is going ahead of schedule and on budget, with connection to the national grid anticipated in the third quarter of 2015.
Firestone has also signed an agreement with Storm Mountain Diamonds, the owner of the Kao mine, in Lesotho, to use the power line.
Firestone Diamonds chief executive officer Stuart Brown said: "Despite adverse weather, increased overburden and the larger rocks encountered at site, the Project continues to make good progress.
"We are also pleased to announce the grid power agreement with Storm Mountain Diamonds who have agreed to contribute half of the expenditure of the power project.
"The costs of additional work crews and the acceleration of certain key areas adopted to meet the revised schedule through to production, now anticipated to be during Q4 2016, are all catered for within the Project’s original US$185.4 million budget."
