Constable Shane Greville on the job in 2013. He has pleaded guilty to a charge of careless driving causing injury.

A police officer who smashed into the back of another car while driving at 91kmh in a 50kmh zone said he had been showing off.

Hawke's Bay constable Shane Greville was off duty and had been drinking with friends on the evening of September 27, 2013, before driving his recently purchased 1967 Chevrolet Camaro along Napier Road.

He accelerated out of a roundabout and overtook another vehicle.

As he did so a woman drove out of her driveway and began travelling in the same direction as Greville.

Greville slammed on his brakes and his car skidded 50 metres before slamming into the rear of the woman's car and shunting it onto the footpath.

The 74-year-old woman, who was not wearing a seatbelt, suffered heavy bruising to her face and ribs and spent a night in hospital.

Police investigators estimated Greville's car was travelling at 91kmh when it hit the woman's car.

When spoken to by police Greville said he was extremely remorseful and had been "showing off".

Residents told police they had heard a extremely loud vehicle travelling at excessive speed.

The woman's car was written off.

Greville pleaded guilty to a charge of careless driving causing injury in the Hastings District Court on Tuesday.

Police prosecutor Neil Coker advised Judge Gerald Lynch the charge had been amended from one of dangerous driving causing injury.

Greville's lawyer Jonathan Krebs said he had an exemplary record as a police officer.

He said Greville had made arrangements to provide the victim with a car until she had replaced hers.

It is understood Greville had bought the rare car for $89,000 a few weeks prior to the crash.

Greville has been on restricted police duties since the crash.

He was convicted and will be sentenced in September.

An internal code of conduct inquiry will occur after sentencing to determine whether he can remain a police officer and if any further action will be taken.