ANZ says the pace of economic growth could slow to about 2% in the months ahead.
The bank's new indicator, the Truckometer, uses traffic volume data published by the Transport Agency as a proxy for future economic activity.
Its heavy traffic index fell 5.35% in September, suggesting that strong expansion in the first half of the year is unlikely to be sustained and signalling a slight contraction in GDP.
The volatile light traffic index, which is supposed to give a six-month lead of economic growth, decreased 2.1%, indicating weakness will persist into the first quarter of next year.
Overall, ANZ says the pace of economic growth will slow from its current 2.5% growth.
ANZ senior economist Sharon Zollner says growth in the second half of the year won't match the first half.
"It's more a question of where the growth in the first half of the year came from," she says.
"I don't think this is an economy that can run at 2.5% at the moment ... the economy will be doing well to turn out 2% growth."