New-car sales fell 9.7 per cent last month, a drop that caught the motor industry by surprise.
New Zealand's August sales were buoyant and forecasters were picking a healthy September market as Japan continued to ramp up production after the March earthquake and tsunami.
Industry analysts point to the Rugby World Cup as slowing buyer interest.
"I don't know if that's the reason but it possibly could be," said Motor Industry Association chief executive Perry Kerr.
He said that despite supply constraints and the slower September the overall market was still ahead on a year-on-year basis.