The number of motorists affected by an ACC blunder over car registrations has blown out to 9000 people as a second critical error has emerged.

It far exceeds ACC's initial estimates just days ago that only a few hundred people had paid the wrong amount to get their vehicle registration.

Since then, ACC has discovered a new fault which it emerges it had known about for at least six months.

The corporation today promised to pay back money to those who were charged the wrong amount for registering cars under a new system which bases cost on the risk level of the car being driven.

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In a statement, ACC said the blowout in the number of those who paid incorrect amounts was down to a second error which ACC had thought it had fixed.

Instead, it bungled the fix of the bungle.

"The error affecting the greatest number of vehicles was identified by ACC late last year and a programme fix was put in place. However, another programme inadvertently overrode this. The issue was not picked up until some customers contacted ACC about their vehicle banding."

The statement said ACC needed a change in regulation - which happens in Parliament - to put cars in the correct bands. "Affected vehicle owners will receive their refunds as soon as possible."

It was unclear whether people would continue to be incorrectly charged until Parliament fixed the mistake.

The corporation again confirmed the problem with data matching variants of cars highlighted in the Weekend Herald. As an example, Toyota Corolla Spacios owners were being charged the higher levy for Toyota Corollas.

"ACC is working urgently with the NZ Transport Agency to resolve the issues and to arrange refunds for all affected car owners. Some refunds will be up to $103.50; most are for lower amounts."

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The changes in cost will save all vehicle owners money after the government cut levies.

The ACC statement said it wanted to "assure people that the vast majority of New Zealand's 2.7 million cars are in the correct band" for vehicle risk assessment.

The statement also said ACC had contracted an external auditor to check its risk rating system was working.

The corporation said any motorists concerned could make contact on 0800 222 776.

NZTA - which handles vehicle registration - has distanced itself from ACC over the blunders. Press secretaries for ministers with transport responsibilities also stepped back from the issue, saying it was for ACC minister Nikki Kaye to respond.

In a statement on Friday, she said it was "very disappointing for ACC to have made this error at a time when we should be celebrating much lower car regos".

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She said the levy changes would see motorists save $40-$130 a year. She said she had sought further information from ACC. "I have also communicated my expectation that they will review their processes and refund anyone who has paid the wrong levy."

The list of affected car models is now online here.