A derailment on the Richards Bay coal line near Ermelo on Thursday is unlikely to result in large tonnage lost, rail and exporter sources said.

The derailment was caused by theft of power cables, a common problem on the South African rail system, they said.

A series of derailments in December and early January plus industrial action cut at least 3 million tonnes from South Africa's exports at the tail end of 2010 and beginning of this year, industry sources said.

South Africa is unlikely to be able to export 60 million tonnes in 2011 and the final total may be even lower if transportation continues to be beset by problems, they said.

There was a minor derailment early on Thursday but repairs are being carried out and no tonnage has been lost yet although there will be some loss as a consequence, one source said.

Transnet Freight Rail had transports 1.45 million tonnes in the week to Thursday and is expected to quickly catch up after the latest derailment.