Fifteen residents of a workers' hamlet of five houses had a miraculous escape when a massive boulder, loosened by the torrential rains, crashed down a mountainside near De Doorns, destroying a De Wet Wynkelder house on Thursday.

The slab of rock, which weighed well over a ton, tore down the steep slope, smashing into the small brick house as precisely as a bowling ball knocking over a pin.

It was the only house among the five that was empty - workers in the four other cottages survived, shaken but unscathed.

Resident Michael Baardman, 25, said the house was empty as it was an old building used for travelling workers from the Piet le Roux winery. "We are very worried. It was lucky nobody was inside."

On Friday the cottage was a mass of rubble and debris covered in mud, which slid down the peak afterwards, half burying the wreck and triggering fears of further collapse.

Baardman said the residents would be evacuated by Monday.