ANZ's Truckometer is showing early signs of the impact of drought on economic activity.
The heavy traffic index, which reflects truck flows along roads selected to provide a close real-time match to gross domestic product, fell 2.7 per cent last month.
ANZ economist Sharon Zollner said the fall in truck volumes was primarily on South Island roads and captured the early end to the dairy season, particularly on the West Coast whose soils are unusually reliant on regular rain.
"Given that the North Island is a more important dairying region the index may therefore be a month ... ahead of itself in terms of the overall GDP impact," she said.
"We suspect we will see North Island truck movements hit in April."