Four people were travelling in a side-by-side off-road vehicle when it crashed into a farm pond but one of the occupants managed to free himself.
Three others, a 66-year-old grandmother and her two grandsons, aged 5 and 10, were trapped in the submerged vehicle and died.
Emergency services were alerted to the incident in Raetihi, in the Ruapehū district, just after 11am yesterday. A vehicle was found submerged in a farm pond, with three people inside unable to be rescued.
A 69-year-old man - understood to be the dead woman's husband - was airlifted to Whanganui Hospital by the Palmerston North Rescue Helicopter in a moderate condition.
He was released last night with minor injuries after being assessed.
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In a statement, police confirmed four people - two adults and two children - were travelling on the side-by-side off-road vehicle at the time of the crash.
"One of the vehicle's occupants, a 69-year-old man, was able to extricate himself from the vehicle," the statement said.
"The remaining three occupants were trapped in the vehicle, which was fully submerged in the water.
"Tragically they were deceased when recovered from the vehicle by emergency services.
"Those involved in the crash were from the same family and police and Victim Support are providing assistance to family members."
Boys' primary school 'completely devastated'
The two children attended Ohakune Primary School, principal Lisa Clark confirmed.
"We as a school are completely devastated by the loss of our precious boys and their dearly loved grandmother. Our thoughts and love are with the family at this extremely difficult time."
Clark asked for privacy for the family, the school and community to give them time to grieve.
The Ministry of Education said it had sent a traumatic incident team to the school. Such teams can help schools as they deal with the impacts of upsetting events such as Sunday's tragedy.
That includes advising how to communicate with children, parents, staff, the community and the media.
"Our thoughts are with the family and the school community at this very difficult time for them," the ministry's deputy secretary Katrina Casey said.
"Our traumatic incident team is currently at Ohakune Primary School, and we will be available as long as is needed."
Volunteer firefighter tried to save family members
The local fire chief has confirmed one of the first emergency responders was the woman's son-in-law, who tried desperately to save the group.
Emergency workers who attended the horrific scene will be provided with support, along with their immediate families.
A son-in-law of the couple is a member of the Ohakune volunteer fire brigade and frantically tried to save the trio from the farm vehicle using a digger to breach the dam walls.
John Hotter, Ohakune Fire Brigade chief, said his crew would be meeting for a debrief tonight to go over yesterday's events.
"It's a tragic accident and we are providing support to the family and the [fire] crew."
He said FENZ had support structures in place to help firefighters who are faced with helping families of their brigade in an emergency response.
The support was not only provided to the affected firefighters but also their families, he said.
WorkSafe has been notified of the incident and is making initial inquiries but has not confirmed whether it will investigate.
Mayor Don Cameron told the Herald the wider community of Ohakune and Raetihi were "deeply saddened for such an awful thing to happen to a well respected family."
That the local volunteer fire brigade and ambulance responders knew the family made the situation even more poignant, he said.
Locals in shock after tragedy hits 'lovely family'
People in the small central North Island community were visibly distraught on Sunday afternoon, describing the family as lovely and well respected.
They had been in Raetihi for about 30 years and had their own business.
The grandparents always loved having their grandchildren over.
"We knew them really well. They're just a really lovely, caring family. Tight-knit," said one person who didn't want to be named.
"It's their highlight to have them out for the weekend."
Dozens of tributes have been posted online for the trio, with the grandmother described as "lovely" and a "beautiful kind lady", who would be "sadly missed by everyone".
"Love and strength to the family, such a tragedy ... my heart breaks for [you]!" wrote one poster. "Such a lovely family too."
Cameron told the Herald the family were well known, well respected and well entrenched in community activities in Raetihi. The couple had five adult daughters.
GP clinic Ruapehū Health, which is based in Raetihi, posted online their "love and condolences to the family of those that passed away".
"To our community, kia kaha and draw on the strength from each other. Kia tau te rangimarie i tenei wā pouri [Peace in this dark time]."