A Paraparaumu man has gone on trial for beating the man who raped his stepdaughter.

A stepfather has gone on trial accused of beating up the man who raped his teenage stepdaughter.

The man, who cannot be named to protect the 15-year-old, has pleaded not guilty to injuring Jason Haward with intent to injure.

Haward, 48, has already been tried and convicted of the rape, at Paraparaumu Beach in April last year. He was sentenced last week to seven years and six months in jail.

Wellington District Court was told on Wednesday that the stepfather saw Haward walking down a road in Paraparaumu several days after the rape.

READ MORE: Beach rapist goes to jail

He leapt from his car and punched Haward repeatedly about the head and face, before headbutting him.

Haward suffered bruises, swelling, and concussion, and lost a tooth. A police officer who attended the scene said he was worried Haward was going to die.

Peter Foster, defending the stepfather, said his client had not intended to injure Haward, but was angry at what he thought was a lack of police action in investigating the case.

Foster said the man planned to restrain Haward until police could arrive.

Two members of the public helped to pull the two men apart.

Colin Riley, who was living nearby at the time, said the man who was hitting Haward kept saying: "He raped my daughter, he raped my daughter, I'm going to kill the b......."

Haward kept saying he had not raped her.

Riley described Haward as helpless to defend himself.

He held on to the stepfather, who nevertheless managed to throw another punch, and headbutted Haward.

John Heenan, who was working in the same street, said he heard a commotion and found Riley and another man trying to separate the fighting pair.

He said the stepfather was aggressive, and yelling about his daughter being raped.

Neither he nor Riley saw Haward strike the man.

Constable Peter Subritzky said he spoke to Haward at the scene. Haward said he was dizzy and felt ill, and that he had hit his head on the concrete.

Subritzky took him to hospital and became worried asHaward began having seizures in the back of the patrol car.

"I became concerned that he was going to die," he said.

They were met by an ambulance and Haward was taken to hospital, where he spent a night.

Subritzky told Foster he did not believe Haward had been faking the seizures.

The trial, before Judge Peter Butler and a jury, is expected to take the rest of the week.