The younger man suffered mental health issues and had returned home from psychiatric care, they said.
"We are the closest family to [the father], he doesn't really have any family around here. We were the first on the scene. Family members heard it. He was just up the road and we went down straight away."
Police and ambulance were called to the property just before 11am after receiving reports that somebody had been shot.
The victim was pronounced dead at the scene by ambulance staff.
He had died from a single gunshot wound.
Police were not looking for anyone else in connection to the shooting.
Wairoa District councillor Charles Lambert said the older man moved back to town 15 years ago to look after his ill mother, who died last year.
"Since he's been living here he's got on with everyone, he knows our community well.
"It's shocking, it's really terrible."
Locals reported seeing up to five police cars heading to the remote town.
Police say they responded to the incident this morning and the Lowe Corporation Rescue Helicopter Service was called to the scene.
It turned back at 11.25am.
Hawke's Bay Rescue Helicopter Trust marketing and fundraising manager Laura Hyde said it turned back "because unfortunately the patient died".
A nearby resident said a number of police cars and officers were in the area, and an ambulance had also been at the scene.
"It will be pretty tight-lipped for a while, I would say," said the woman.
One woman said she saw five police cars heading towards Mohaka and that locals were saying there had been a shooting.
She said an ambulance also left, but had returned.
The police cars sped past about 11.30am, she said.
The incident comes after Mental Health outpatient Ross Bremner is believed to have stabbed his mother, Clare Bremner, to death and left his father, Kevin Bremner, fighting for his life last week.
Ross Bremner's body was found four days later alongside those of elderly couple Mona Tuwhangai and Maurice O'Donnell at their Te Kuiti farmhouse.
Psychotherapist Kyle MacDonald said it was worrying that there had been another case involving someone who had been receiving psychiatric treatment. "There's clearly a lack of resource when it comes to people receiving the treatment that they need."
MacDonald said he hoped it wasn't a case of someone not being in care for when they should be.
"According to the Health Minister there's no crisis in mental health ... they keep reassuring us that they're doing something, but everyone in the trenches, whether it's the clients or the workers, they're telling us that it's getting worse."
