British platinum producer Lonmin said the breakdown of its number one furnace earlier this month would increase the platinum in process by about 20,000oz by September 2009.
An inspection of the furnace revealed the matte run-out was caused by a water leak in one of the lower copper cooling waffle units.
The unit is being replaced and the matte is expected to be tapped again in 30 days, Lonmin said.
The estimated cost of repairing the furnace and running the Pyromet furnaces, deployed to mitigate production losses, is about $4m up to September this year.
Lonmin has launched a full investigation into the cause of the leak.