The economy had a "soft underbelly" last year and did not get much better in the final three months of 2012, according to a bank report on traffic flows.
The ANZ Truckometer uses figures from the NZ Transport Agency on traffic volumes for trucks and lighter traffic on a number of key roads, to give an indication of the state of the economy.
Promising lifts in the Truckometer in October and November were erased in December.
"The absence of a clear trend flags a stagnant economy," ANZ said.
The heavy traffic index, which monitors trucks above 3.5 tonnes and shows a close link with GDP, fell 5.4 per cent in December, wiping out gains seen in the previous two months.
"The heavy traffic index flagged an economy with a soft underbelly in the September quarter of 2012 and now suggests only modest improvement in the December quarter," ANZ said.
Official figures showed the economy was almost treading water in the September quarter, with growth of just 0.2 per cent.
The light traffic index, which covers mainly cars and vans, fell 1.3 per cent in December, seasonally adjusted. The index typically gives a six-month lead on GDP growth, but is volatile.
"The trend in this index is also subdued," ANZ said.
Weakness in the underlying traffic volumes indicated an economy with some persistent weakness, despite positive signs from sectors such as housing and confidence surveys, ANZ said.
Yesterday, the NZIER Quarterly Survey of Business Opinion showed activity in the December quarter surging to its best level since mid-2007.
Businesses were much more optimistic about the general economy at plus 19 per cent, compared with negative 1 per cent in the September quarter. A reading above zero means there are more optimists than pessimists.