The Police Association says proposals to cut the police vehicle fleet make no sense when a shortage of cars is already affecting officers' ability to do their work.
The police say they may cull more than 300 vehicles from their fleet, as part of a drive to cut $21 million from their billion-and-a-half-dollar annual budget.
The force currently has 3,397 vehicles and the Deputy Commissioner, Lynn Provost, says a 10% cut would save $2.4 million without compromising services.
The Police Association says the move flies in the face of Government promises to get tough on law and order.
Association president Greg O'Connor says officers are already frustrated by a lack of cars and cutting the vehicle fleet will further reduce their ability to carry out inquiries and interview witnesses, as well as damaging the public's opinion of police.
The Labour Party's spokesperson for police issues, Clayton Cosgrove, says there is no point in putting additional police on the frontline if they don't have the vehicles they need.
Police Minister Judith Collins denies there will be a shortage of frontline patrol cars if the number of vehicles is cut.
Ms Collins says her preference is for reductions in vehicles such as police vans, rather than frontline patrol cars. She says the Government allowed for 43 extra police cars in the Budget, delivered in May, to ensure the numbers are adequate.
The minister says Police Commissioner Howard Broad has assured her that the safety of staff and the public will not be compromised.