Some 36,000 vehicles will jump from being the riskiest to the safest in a crash under ACC's risk rating system - saving each of those motorists at least $132.92 for their 2016/17 regos.
And more than 25 per cent of all vehicles on the road - 774,565 - will change rating bands from July 1.
But the Government says it will not refund even the 1.3 per cent of people most affected - those who paid a total of $4.78 million more last year than they will this - because the changes are due to improvements to the system, not wrong information.
Owners of the 44 most affected models - which include Toyota Hiaces, Kia Sportages, the Nissan Dualis, Hyundai Sonatas, Subaru Imprezas, Mitsubishi RVRs, Nissan Qashqais manufactured in certain years - will each save $132.92 for petrol vehicles and $141.30 for non-petrol from July as their vehicles move from band 1 to band 4.
ACC rolled out the new vehicle licensing system last year with band 1 representing the vehicle posing the highest risk to road users and band 4 being the lowest risk.
Meanwhile 2900 vehicles have dropped from having the best risk rating to the worst. Those owners will be pinged between $8 and $16 more for their 2016/17 registration.
On average the motor vehicle levy will fall from about $195 to around $130 per vehicle from July 1, but this alters depending on the vehicle's risk-rating band.
Andrea and Terry Hill were shocked to learn last year their 2011 Nissan Qashqai carried the highest risk in a crash. The Lower Hutt couple had spent hours choosing the safest car, finally picking the 4WD, brand new for $40,000 in 2010.
When they got the new registration notice last year telling them it was in the worst category, Mrs Hill contacted NZTA and ACC Minister Nikki Kaye's office and were told the information was correct.
"We were shattered."
But now their vehicle has gone from being the worst to best rated and their registration will be much cheaper.
Mrs Hill accepted mistakes could be made, but was angry ACC refused to own up to it and refund people.
"How can our car have been deemed safe, then deemed unsafe and then safe again?"
But ACC said the 27 per cent of vehicles reclassified this year was larger than normal due to two years of crash data - 2014 and 2015 - being entered into the system.
When the system started last year it was based on 2013 Monash crash data due so public consultation could happen before the general election and vehicles were classed based on the year of registration.
Ms Kaye said the 20,000 people subject to a technical error last year had been offered refunds, but denied the latest reclassification of a quarter of cars was due to a mistake. "Of course we will refund where there are errors, but every time the Government makes a decision on a policy they don't retrospectively refund people because you develop a system and make it better ... no one had that database. [It] didn't exist."
"There will be a proportion as a result of new database we have created we think - separate to new crash data that has come in and separate to new cars in the fleet - that this car better fits within this band." The Hill's Nissan Qashqai was one of these vehicles.
ACC paid $30,324 for crash data from Monash University's Accident Research Centre and the cost is expected to rise in future. As of March 31, it also spent $1.028 million on reviewing the new system and had budgeted a further $847,000 for the cost of maintenance and the ongoing review/development of its vehicle risk rating system in 2016/17.
Labour ACC spokeswoman Sue Moroney said there could not be such a big jump based solely on new crash data and said the original data was "fundamentally flawed".
She said affected vehicle owners should be refunded.