Transport officials have called in debt collectors after drivers racked up more than $500,000 in unpaid tolls on the Northern Gateway toll road in just six months.
The New Zealand Transport Agency on Monday released the operating figures for the second six months of the new tunnel and road near Puhoi, north of Auckland.
The agency has an internal collections process, but has now called in debt collectors to go after the worst offenders.
It says one driver amassed more than $700 of unpaid tolls in the six months.
The Transport Agency says from July, those who persistently avoid paying tolls will have a $40 fine added to every unpaid toll.