A relative of the trio who drowned when their farm vehicle plunged into a pond tried desperately to free them with a digger.

However, it was too late and there was nothing he or other emergency service workers could do to help the woman and two children, who were believed to be her grandchildren.

The tragedy, which struck on a 74.5ha family farm in Raetihi at 11am, has left the small central North Island community in shock.

A 69-year-old man, believed to be the woman's husband and childrens' grandfather, was flown to hospital at about 2.40pm. It is unclear if he was in the vehicle, injured trying to help after it went into the water or suffered a medical event later in the day.

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A Whanganui hospital spokesman said the man was treated for minor injuries in the emergency department and released this evening.

Mayor Don Cameron told the Herald the couples' son-in-law is in the local volunteer fire brigade and raced to the farm upon hearing the news.

He grabbed a digger and tried to breach the walls of the pond to free those trapped inside.

"One of the son-in-laws managed to get a digger and cut into the dam to release the water out so people could get to them but it's (6m) deep, it's a deep dam. But unfortunately they were too late."

He described the vehicle as a side-by-side, which is like an off road four-wheel drive vehicle that isn't completely enclosed.

"I know it was a side by side vehicle and they basically couldn't get out. It's so tragic."

A hearse leaves the Raetihi farm property this afternoon. Photo / Lewis Gardner
A hearse leaves the Raetihi farm property this afternoon. Photo / Lewis Gardner

Cameron said the family are well known, well respected and well entrenched in community activities.

The couple had five daughters and the children were believed to be primary school aged.

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Police said they were called to a report of a vehicle underwater in a farm pond.

"Tragically the three people were deceased when recovered from the pond by emergency services," police said.

"Police are working to support the family of those killed. WorkSafe has been notified."

Locals were visibly distraught this afternoon as most were still absorbing the news that three people had died.

They described 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."

One couple were too upset to talk, simply stating the tragedy as "sad" and that the shock of what happened was still sinking in.

Another described the deaths as "just unbelievable".

One resident said they knew something bad had happened when they saw the rescue helicopter land at the property after a flurry of police and St John Ambulance vehicles rushed to the scene.

They all described Raetihi as tight knit and although no names were being mentioned in social media, locals knew who had been affected.

One neighbour said the "pond" referred to by police would more likely have been the farm dam, which supplies water for stock/animals.

"Every farm had them around here."

There had been a lot of rain in the area in recent days and it's likely the ground would have been slippery.

A Palmerston North Rescue Helicopter spokeswoman said a man was flown from the scene to Whanganui Hospital in a moderate condition just before 2.40pm.

She could not give more information about his condition.

Tassia Allum, an employee at Caltex Raetihi, said she heard sirens and saw two ambulances, a fire engine and a police car all heading up State Highway 4 at 11am.

The property is just out of the town and the owners operate a farm improvement business there, offering diggers, trucks, bulldozers and fencing materials.