The pressure is mounting on road police, as fewer staff try to tackle a bigger job.

Nearly 100 dedicated road police positions have been lying vacant since 2016, while the road toll rose 50 per cent in the past five years.

Crashes on New Zealand roads killed 253 people in 2013 - the lowest on record since 1950 - but five years later it had risen by 49.8 per cent to 379 in 2017.

During that same period the number of dedicated road police fell from 1063 to 976.

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Police Association president Chris Cahill said about 10 per cent of road policing roles haven't been filled and it means other officers are having to pick up the work left by the vacancies.

"Members involved in road policing suggest that they are carrying more of those vacancies than other areas of the police. This needs to be looked at it when you look at the increasing road toll.

"It equates to roughly 10 per cent of the dedicated road policing staff, which we would say is too high in any one area and unsustainable long-term. We'd certainly like to see those vacancies filled."