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Constable Greg Nolet, of Wanaka, said the area had received its first rain in ''a long time'' which meant dust and oil that had built up during the dry weather had become slippery.
Const Nolet said the campervan had hit the guardrail on the roadside, preventing it rolling down a steep drop, before tipping over on the road and blocking one lane.
It was a ''low-speed roll'', he said, and pointed to evidence of another incident which appeared to be recent.
Tyre marks indicating the driver of another vehicle had lost control were seen going up then back down the base of the hill on the other side of the road.
Constable Hugh O'Reilly, of Queenstown, said the campervan was driven by a 22-year-old Dutch woman travelling from Wanaka to Queenstown on a downhill section of the road.
Inquiries were continuing, he said.