Two motorcyclists were treated for minor injuries after they took evasive action to avoid a rock on a winding section of State Highway 6, between Makarora and the Neck, about 11am yesterday.
The motorcyclists - a woman from Waihola and a man from Balclutha - were with a group of Harley-Davidson riders.
Constable Pete Reed, of Wanaka police, said the female motorcyclist had been first to encounter the rock, which he described as "haggis-sized", as she came around a corner.
Const Reed said the woman broke her right elbow and was bruised in the fall from her machine, while the man travelling behind her was concussed and broke ribs.
They were taken to Wanaka Medical Centre by St John for treatment.
The section of State Highway 6 between the Neck and Makarora is known for rock falls from the schist cliffs above the road.
Const Reed said it appeared the accident was because of "Mother Nature" and no charges were being contemplated at this stage.
The motorbikes were believed to have only minor damage.
Wanaka police are tracing the driver of a vehicle that reportedly drove on to the pavement of Meadowstone Dr about 12.30pm on Saturday, threatening the safety of a woman and her baby.
Constable Pete Reed said the driver possibly faced a dangerous driving charge.
