A 6-year-old Stokes Valley girl died after losing control of her quad bike and hitting a fence, north of Masterton, on Sunday afternoon.
The incident occurred at the venue for the Jayden Kilmister memorial trail bike ride at Te Ore Ore Bideford Rd.
It is believed the girl was not taking part in the memorial event.
Senior Sergeant Jymahl Glassey said the death was "an extreme tragedy" and officers who attended were shaken. "A young girl has hit a fence and suffered fatal injuries." She was wearing a helmet and safety gear at the time and was riding on a gravel driveway, not a track, he said.
St John paramedics, who were on site as part of the event, administered CPR but were unable to resuscitate the girl. She was the sole occupant of the quad bike, Mr Glassey said.
He said the girl had been riding a 50cc child's bike, "smaller than the average farm bike. It's made for children, and there are many of them on the market. But all machinery is dangerous - you need to take care."
People needed to be aware that even a 50cc quad bike could be dangerous, Mr Glassey said.
"People should be mindful that they are a machine, just like a car, and can have accidents."
Mr Glassey said police were speaking to organisers about the sequence of events and were supporting the family.
"We can't say what's going on in their minds. It will be hard for them to deal with. We are not looking at this in terms of laying of charges at this stage, we just want to look after the family."
The death has been referred to the coroner and the serious crash unit is investigating.
The trail bike ride was held to commemorate Jayden Kilmister, a 15-year-old who died in 2009 after a gust of wind blew him off the back of a ute, along with furniture he was moving. All funds raised went to the Jayden Kilmister Memorial Trust, to aid local families in times of similar need, and organisations including the Westpac rescue helicopter and SPCA.
Federated Farmers Wairarapa president Jamie Falloon, who was at Sunday's event but did not witness the accident, said people were devastated.
"It was just a terrible accident. Kids ride horses and motorbikes. Kids do a lot of stuff and there's always risk involved, and we just hope that we never see the consequences. Wrapping kids in cotton wool is not the answer."