Hundreds of motorists are falsely registering their cars as ambulances, avoiding more than $200 in fees.
The NZ Transport Agency said last month's figures showed 2681 vehicles were registered as ambulances.
But St John and the Wellington Free Ambulance services have only 705 registered ambulances between them - meaning up to 1976 private vehicles could be falsely registered.
An ACC levy exemption for ambulances means it costs only $52.11 a year to register a non-commercial ambulance, compared with $280.55 for a petrol-driven passenger car - a difference of $228.44.
Do you know of any cars registered as ambulances? Email us here.
The difference is even greater for commercial vehicles, which cost up to $590.78 to register.
The total loss in levies to ACC is at least $392,500 a year.
Auckland had the most ambulance registrations with 309 - many more than the 186 ambulances in St John's northern region, from Northland to Coromandel.
It was followed by Hamilton with 288, Christchurch with 270, Manukau with 147 and Hastings with 107.
Transport Agency spokesman Andy Knackstedt said anyone who falsely licensed a vehicle as an ambulance was effectively rorting ACC.
The agency did not seek evidence of a vehicle's use when it was registered, but vehicle owners who deliberately provided false information could be fined up to $1000 and risked losing insurance cover if they were involved in a crash.
Mr Knackstedt said that in the past year, 224 vehicle owners had changed their registration type from ambulance to private vehicle.
The law allowed the Transport Agency to require owners to provide evidence of vehicle use, Mr Knackstedt said. But this could increase costs and cause inconvenience, and so would have to be balanced against the scale of the problem.
ACC spokesman Glenn Donovan said levies covered the cost of helping people injured in crashes. "Choosing to register a vehicle incorrectly is therefore depriving ACC of funds it needs."
Next trending article: Robin Williams Emmys tribute criticised as 'racist'