The latest ANZ truckometer fell 1.9% in June, a considerable drop after a sharp increase in May.
The truckometer uses traffic volume data from the New Zealand Transport Agency and matches the data from selected roads to GDP growth six months later.
It is designed to predict what economic activity will be like in six months.
ANZ senior economist Sharon Zollner told NBR ONLINE after a 4.1% increase in May, a decline in June was inevitable.
"It was encouraging it didn't fall by more.
"It held on to about half its gains which took it back to the trend that had been evident until about the middle of last year, which is not spectacular.
"It is consistent with fairly sluggish growth in the economy."
The truckometer uses data from heavy and light traffic, with heavy vehicles reflecting current activity and light traffic predicting consumer spending six months out.
In June, heavy traffic fell following a very strong May, which was likely due to a strong end to the dairy season.
The heavy vehicle data suggests growth for Q2 of 0.5% to 1%.
In contrast, the light traffic index from late last year, when it dropped in the latter months, suggests Q2 growth will be negative.
While the outlook for Q2 is not sensational, the truckometer predicts growth to pick up later this year.
It suggests 1% growth quarter-on-quarter for the final three months of 2012.