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Hundreds of motorists are falsely registering their cars as ambulances, avoiding paying more than $200 in fees.
The New Zealand Transport Agency (NZTA) said 2681 vehicles were registered as ambulances, as at July.
But St John and 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.
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 greatest number of ambulance registrations with 309 - far more than the 186 ambulances in St John's entire northern region, which spans Northland to Coromandel.
It was followed by Hamilton with 288, Christchurch with 270, Manukau with 147, Hastings with 107 and Waitakere with 106.
NZTA spokesman Andy Knackstedt said vehicle owners were required to licence their vehicles correctly, and anyone who falsely licensed a vehicle as an ambulance was effectively rorting ACC.
The agency did not seek evidence of a vehicle's use at the time of registration. However, when it became aware of a falsely registered vehicle, it sent a letter warning the owner of the potential consequences and asking them to correct the registration type.
Vehicle owners who deliberately provided false information could be fined up to $1000, and also risked losing insurance cover if they were involved in a crash.
Mr Knackstedt said warning letters were sent out regularly. In the past year, 224 vehicle owners had changed their registration type from ambulance to private vehicle.
The law did allow NZTA to require owners to provide evidence of vehicle usage, Mr Knackstedt said.
"But this could potentially increase the administration costs and the time required to process vehicle licensing transactions for a large number of vehicle owners, and this additional cost and inconvenience would need to be balanced against the relatively small scale of the problem of vehicles being incorrectly licensed."
ACC spokesman Glenn Donovan said levies covered the costs of supporting people injured in crashes on public roads.
"Choosing to register a vehicle incorrectly is therefore depriving ACC of the funds it needs to provide this support to people who need it."
Because ACC was a no-faults scheme, it covered treatment regardless of whether someone was involved in legal or illegal activity at the time of an injury.
"This could potentially include those who incorrectly register their vehicle, as well as their fellow New Zealanders."
A Wellington Free Ambulance spokeswoman said ambulances were a precious resource that did an important job.
"We fundraise around $4 million each year to keep the wheels turning on our free service. Reduced registration is of significant importance to us as a charity."
St John did not wish to comment on the registration numbers.
- By Matthew Backhouse of APNZ