A 61-year-old man has been killed by his son's dog while his wife tried to save him at a home in Mill Park, in Melbourne's north.

Key points: The dog was still attacking the man in the backyard when police arrived

The dog was still attacking the man in the backyard when police arrived Police fired several shots to distract the dog and stop the attack

Police fired several shots to distract the dog and stop the attack The dog, believed to be an American Staffordshire terrier, was removed by rangers

Police fired a number of shots to try to distract the animal and stop the attack when they responded to a triple zero call from a neighbour at about 6:40pm yesterday.

When police arrived at the property, they found the dog, believed to be an American Staffordshire terrier, attacking Leo Biancofiore in the backyard.

Mr Biancofiore, who police said was on crutches, died at the scene.

His 58-year-old wife Donna was taken to hospital with serious but non-life-threatening injuries.

Neighbours said the dog belonged to Mark Biancofiore but was kept at his parents' house. ( Facebook )

Senior Sergeant Glenn Parker said it was believed the dog started attacking Mr Biancofiore inside the house and Mr Biancofiore's wife helped him move outside to get away from the animal.

Neighbours reported hearing screams and gunshots.

The dog was taken away by rangers and put down.

Locals said the dog was called Junior and belonged to the couple's son Mark, but was kept at his parents' house.

One neighbour said the attack was a "tragedy waiting to happen", as the dog could be aggressive at times.

The City of Whittlesea said the dog was not registered and the council had not received any reports about the dog prior to the attack.

Leo and Donna Biancofiore had lived in the street for almost two decades. ( Facebook )

Senior Sergeant Parker said the attack appeared to have come "out of the blue".

"They've got no idea what's actually started it off," Senior Sergeant Parker said.

"It's an older dog — it's quite familiar with most of the members of the family and my understanding is it's completely out of character for the dog."

'A beautiful man'

Neighbours described Mr Biancofiore as a "nice and lovely" man who "spoke to everyone".

Carmen Baldwin, who lives nearby, said the news came as a shock.

"It's very sad, he's a lovely bloke," she said.

Police said they were not aware of any complaints about the dog's behaviour prior to the attack. ( ABC News: James Hancock )

"He was a beautiful man, very kind, he always waved at me, very nice bloke."

Neighbours said Mr Biancofiore had been unwell for some time and used a wheelchair and a mobility scooter to get around.

The couple had lived in the house for about 18 years, they said.

Police fired a number of shots in a bid to distract the dog. ( ABC News )

Because a police firearm was discharged, professional standards command will investigate the incident.

Police will prepare a report for the coroner.

'Breed alone' not a predictor of aggression

RSPCA Victoria's head of operations, Tegan McPherson, said there were a number of reasons why a seemingly healthy dog could suddenly attack.

"It can be a range of things such as genetics, its learning experience and training as well as its environment," Ms McPherson told ABC Radio Melbourne.

"Fear is generally the underlying emotion associated with aggression in healthy dogs.

"Breed alone is not a reliable predictor of aggressive behaviour."

Warning signs of stress in the animal could include yawning, a high tail, a tail tucked between its legs, the dog's ears being back, or the animal suddenly freezing, she said.