An injured man broke a police officer's nose as the officer tried to help restrain him in an ambulance in Hamilton last night.

Inspector Karen Henrikson said the Senior Sergeant and the 19-year-old Wairarapa man accused of injuring him had been taken to Waikato Hospital for treatment.

Ms Henrikson said the police officer's nose had been broken in two places - and he was celebrating his fortieth birthday today.

"He turns 40 today - a great birthday present."

Ms Henrikson said the 19-year-old was expected to be interviewed today in relation to the incident and could be charged.

Police were called to reports of two men fighting in Clyde Street, Hamilton East, about 9.15pm, finding an unconscious man lying in the street.

But when the man - who was "highly intoxicated" - was being treated by ambulance staff, he became aggressive "to the point my staff became concerned enough to restrain him," Ms Henrikson said.

The officer was struck in the face by the back of the man's head, suffering fractures in two places and a mild concussion.

"They were trying to quell his aggression in the ambulance and [the officer] was behind him and he flicked his head backwards, so it was a backward headbutt to the nose."

She said the man was heavily intoxicated with alcohol and drugs and told police today he could not recall the incident.

Ms Henrikson said the nature of policing meant that officers were at risk from such incidents.

But she said it was concerning that many cases involved offenders under the influence of drugs and/or alcohol.

"We need to look at where our choices are taking us and I can assure you if those choices take you in a direction where our staff are injured the consequences can be very severe indeed."