Robert Rata Moka (46) was accused of punching Rachelle Faumuina during a road rage incident in Mangere in October 2014.
He was also accused of intentionally damaging the black Ford Kuga she was in and driving recklessly.
Moka and his daughter Aroha Kahui, 20, were jointly charged with intimidating Mrs Faumuina by threatening her.
Moka was found guilty in Manukau District Court today on charges of male assaults female, reckless driving and intentional damage.
Earlier this week, Mrs Faumuina told Manukau District Court her rugby-star husband was speaking at Otahuhu College on the day in question and she had gone to get a drink to kill time.
While letting a bus merge in front of her, Mrs Faumuina said the driver behind became impatient and repeatedly tooted their horn.
"I put my hand out and did the middle finger and it just started from there," she said.
Mrs Faumuina said her son, who was a 1-year-old at the time, had been seated in the back of the car.
She pulled over and said the duo came to a stop beside her, yelling that she was "messing with the wrong people".
Mrs Faumuina told Judge John Bergseng she approached their vehicle to explain the incident with the bus and apologised for the one-fingered gesture.
"Get back to that car before I punch the baby out of your stomach," Moka allegedly said.
She accelerated away and told the court she clipped their wing mirror as she left.
Eventually she pulled over again outside a shop but Mrs Faumuina said she spotted the other car approaching at speed, eventually bumping into the side of hers.
"They both came running to the car and the father was just yelling 'you're going to pay. We're gang related'. He grabbed my head with his left hand and punched me right in my forehead," she said.
Moka's lawyer Esma Brown said the reported threats were fabricated but Mrs Faumuina maintained they were "very true".
Ms Brown said her client accepted he had punched Mrs Faumuina but that it was done legally in the course of citizen's arrest.
"I put it to you he was upset at the time and as he said to you, you'd just run over his daughter and he didn't want you absconding," she said.
Mrs Faumuina denied trying to leave the scene.
Meanwhile, Kahui -- representing herself -- suggested there was a lot of anger coming from the complainant.
She claimed she was run over and had the tyre marks on her legs "to this day" to prove it.
Judge Bergseng today said he accepted Mrs Faumuina had hit Kahui with her car but rejected the defence's claim that Moka had been justified in hitting Mrs Faumuina's car then punching her in a citizen's arrest.
He said Mrs Faumuina was seated in the car, pregnant and there was no evidence she was seeking to drive off.
Moka will be sentenced on the charges on June 23 following a Provision of Advice Report and updated victim impact statement.
Moka was today found not guilty on a charge of threatening behaviour. He and Kahui were also found not guilty on the charges of intimidation, because independent witnesses did not hear the threats the pair were alleged to have made in relation to the charges.
Kahui was convicted and discharged on a charge of driving while forbidden which she had previously pleaded guilty to.
