Any person should be able to defend themselves or their family without risking a conviction, the Sensible Sentencing Trust says.
The trust is stepping in to help a Paraparaumu man with his appeal after a jury found him guilty of bashing his stepdaughter's rapist.
The man, who cannot be named to protect his stepdaughter, beat up the rapist, Jason Haward, last year.
A couple of days before, the man had come outside to find Haward grabbing at his 15-year-old stepdaughter as she crouched naked beside a car.
Haward was found guilty of raping the girl.
The father and his family beat the man up at the scene, but were not charged for that incident.
Police arrested Haward and released him later that day.
However, a few days later, the man and his partner were driving when they saw Haward walking down the road.
The man said he had evidence from the rape clinic in the car and was hoping to hold the man there until police could arrive and he could give them the evidence.
He said he "lost the plot" when the man began calling his daughter a name and refusing to stay put. The man began hitting Haward but was dragged away by members of the public.
Haward suffered four seizures after the assault.
A jury found the man guilty earlier this month of injuring with intent to injure.
Sensible Sentencing Trust child abuse spokesman Scott Guthrie said the trust will be supporting the father in appealing any conviction.
"Our interest simply is, in the name of open and transparent justice, we don't believe that anybody who defends their family or themselves should really be up for a conviction," Guthrie said.
The trust has "gone in to bat" for people in the past before, such as Matthew Oates, who was charged with murder for shooting a man who was holding a shotgun to his partner's head during a home invasion.
The murder charge was eventually dropped.
In the Kapiti case, Guthrie said the man was only doing what any father would do.
"This particular case, I think any father would defend their child in the way that he did, stepdaughter or natural child."
The trust would help the man by trying to get the judge to overturn a conviction, or it would work through the appeal process with him, Guthrie said.