ANZ Bank's Truckometer measures traffic flows on certain roads and calculates the economic impact.
The heavy traffic index, which is an indicator of the economy as it is performing now, fell 2.1 percent in May after rising 0.5 percent in April.
But ANZ said it can still discern an upward trend, discounting monthly volatility.
And the light traffic index, which predicts growth 6 months ahead, rose 0.2 percent in May, it's third consecutive monthly lift.
Economist at ANZ Sharon Zollner said the outlook is still strong although growth wont quite match the first quarter.
"That wouldn't be surprising as we're looking at strong growth in the last quarter of last year plus the first three months of this year. So it wouldn't be a bit surprising to see an easing of that growth pace in the second quarter of this year."