Geologist John Compton told the Cape Times that the section of mountain in question was made up of weathered granite and was very unstable.

He said while engineers would probably come up with a design to make the section more stable, this was likely to add to the cost.

"The road cuttings along Chapman's Peak reveal a very thick, rotten granite that is normally covered by vegetation. It's thick, yellow, and crumbly.

"It's a classic problem when building a road: you artificially produce a steeper slope, which is not as stable as it was before."

He estimated the project's cost could rise from about R53 million to R80 million.

Provincial transport department spokesman Siphesihle Dube told the newspaper he was not aware of the "landslide", but would ask toll road concessionaire Entilini what had happened.

Earlier in the year, Hout Bay residents lost an urgent court application to halt the toll, claiming no permission had been granted for the control building to be erected within the Table Mountain National Park, a World Heritage Site.