"My concern aside from obvious physical harm of them, is that this girl is being treated so badly that she doesn't feel she has any worth. She is someone's precious daughter that should be being treasured and not harmed."
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Duncan was parked at Pak'nSave in Kaitaia when he heard screaming and arguing coming from a nearby car on SH1 which had stopped in its lane.
A young female was in the back opening her door trying to get out and the male reached over and with a closed fist punched about five times, he wrote.
The driver then took off at speed northbound followed by Duncan and several other vehicles had also stopped to help.
At some stage the distraught woman managed to get out of the car.
Before she was picked up by another vehicle, believed to be that of her family, Duncan told her he would take her to safety, but she refused to go with him.
"I told her that she was worth more than this. She was very distraught and said she wasn't worth anything."
He then grabbed the registration of the car, and intended to confront the driver, but he drove off.
"Girl, if you're reading this, you are worth something! Duncan wrote.
"Young man, if you're reading this, you need to know that it's a man's job to be a protector and provider and to be a place of refuge and safety for your woman and child. Get the help you need to stop this path you're heading."
Kaitaia Women's Refuge Whare Timatatanga Hou Ora Chief executive Waimaria Veza said the woman was traumatised and had some physical injuries but they were not life-threatening. The baby was not hurt.
Veza said she has seen an increase in domestic violence over the last three years.
"We don't know if the increase is from more people committing domestic violence or that more people are reporting it - I think it's a bit of both," she said.
"People are speaking out more publicly about domestic violence, it's no longer clandestine. Everybody talks about it now and knows that behaviour is not okay."
Kaitaia Senior Sergeant Russell Richards said there were seven family harm incidents reported to police on November 2 which was "an average day for Kaitaia police".
Family harm is a serious issue in Te Hiku, and can involve anything from yelling through to physical violence, he said.
He advises people to phone police if they witness domestic violence.
"If people have mana in the community to approach young couples to make people safe, if they feel comfortable with that, then certainly, but police need to be called as well to ensure they're connected to the right services to prevent further incidences."