ANZ's Truckometer, a measure of economic activity using traffic data, slowed last month after advancing solidly in June, sending a possibly worrying signal about the strength of New Zealand's economy.
The bank said a 5.7 per cent fall in the July heavy traffic index was "conspicuous as it runs counter to most other economic indicators and anecdotes".
The light traffic index was little changed, ANZ said.
ANZ senior economist Sharon Zollner said the Truckometer had already been suggesting weaker economic growth in the second quarter than the first.
"The sharp fall in July represents a poor start to the third quarter, though it is only one month of three," Zollner said.
Meanwhile, ANZ said annual growth in the heavy traffic index - on a three-month rolling average - slowed to 1.9 per cent from 4 per cent, suggesting annual GDP growth may slow.
"The heavy traffic index is the only indicator that is currently so pessimistic."
However, Zollner said the light traffic index, which gives a six-month lead on the economy, fell 0.6 per cent in July, largely holding on to the rise it registered in June.
"This index has had a very strong upward trend in recent years, and suggests the economy is on a roll. It is not sending any warning signals about an impending slowdown," she said.
"Given the weight of other positive evidence regarding the state of the economy we are not reading too much into the turn in the heavy traffic index at present. The data is volatile, and a solid bounce-back in August seems the most likely outcome."
Zollner said mid-winter was a "trough" for truck movements and timing or weather events could be at play.
"But if August doesn't bring the expected solid bounce-back we'll start to wonder what's going on under the hood of this economy."