ANZ's heavy traffic index fell 2.9 per cent last month, seasonally adjusted, reversing most of its gains over September and October.
The index reflects movements of trucks and tankers on 11 roads selected because historically they provided a good indicator of economic activity.
"The trend remains upward but may beshowing early signs of flattening out," said ANZ economist Sharon Zollner.
But some deceleration was to be expected. "This economy can't sustain growth above 3 per cent for too long."
Zollner suspects the strength of heavy traffic flows in September and October reflected a good start to the dairy season and possibly a post-election return to normality "but neither of those as growth drivers is going to be sustained for more than a month or two".
Meanwhile, the companion light traffic index, which reflects movement of cars and vans and is a pointer to activity six months ahead, fell 1.4 per cent last month, its steepest drop since the middle of last year.
However, that index is still at levels historically consistent with annual gross domestic product growth above 4 per cent six months down the track, similar to the results of the bank's surveys of business and consumer confidence.
ANZ does not expect that to eventuate. "However, the light traffic index certainly indicates that the domestic economy still has plenty of momentum," Zollner said.
