Purebred or mixed-breed Staffordshire terriers have killed at least four people in Australia in the past six months.

Key points: An American Staffordshire terrier which killed a man and injured his wife in Melbourne on Wednesday night has been put down

An American Staffordshire terrier which killed a man and injured his wife in Melbourne on Wednesday night has been put down Purebred or mixed-breed American Staffordshire terriers were responsible for three other fatal attacks in NSW this year

Purebred or mixed-breed American Staffordshire terriers were responsible for three other fatal attacks in NSW this year The RSPCA says breed is not a reliable predictor of aggressive behaviour

In the latest case, 61-year-old Leo Biancofiore died after his son's American Staffordshire terrier attacked him in his home at Mill Park in Melbourne's north on Wednesday night.

His 58-year-old wife Donna, who tried to save her husband from the animal, was taken to hospital with serious but non-life-threatening injuries.

In June, a man was found dead in his unit at Nowra on the south coast of NSW. Detectives later said they believed the 51-year-old man had been mauled by his Staffordshire bull terrier after suffering a fit.

The man's injuries were so severe investigators initially treated the man's death as a homicide.

Donna and Leo Biancofiore were looking after their son Mark's dog. ( Facebook )

In May, 72-year-old Rosemary O'Reilly died in hospital after her Staffordshire terrier-Rhodesian ridgeback cross reportedly turned on her and her 74-year-old husband at their home at Wilton, south-west of Sydney.

In March, a 40-year-old man died in hospital five weeks after he was attacked by two American Staffordshire terriers at home at Tregear in Sydney's west.

A 30-year-old woman and a 10-year-old girl were also attacked by the dogs and were taken to hospital with minor injuries.

Victoria Police and the RSPCA are continuing to investigate Wednesday night's attack.

Because police fired shots in an attempt to distract the dog and stop the attack, professional standards command is also investigating the incident.

The dog, called Junior, belonged to the couple's son but had been kept at his parents' house for some time.

A 40-year-old man was fatally attacked by two American Staffordshire terriers in Tregear in Sydney's west in January. ( ABC News )

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

It was removed by rangers and put down on Wednesday night.

While a neighbour said on Thursday the dog could be aggressive at times, police said the attack appeared to have come "out of the blue".

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

In a statement posted on its Facebook page, the American Staffordshire Terrier Club of Victoria said its thoughts were with Mr Biancofiore's family and the others affected by this week's attack.

"Any incident needs to be treated on an individual basis and not under an umbrella as to try and tarnish a particular dog breed," the group said.

Don Campbell, president of the club, said he was "shocked" by what had happened.

"The breed is not human-aggressive," he said.

"It's a very people-orientated breed. If it's a well-bred American Staffordshire terrier it's so out of character."

Mr Campbell said he has owned American Staffordshire terriers for more than 20 years and never had a problem.

"My daughter and my son were raised around them, my grandkids are around them all the time and there's never been an issue with them," he said.

"It's a dog I trust. They're a loyal dog, they're a very people-orientated dog. They just want to please people."

American Staffordshire terriers could make "wonderful family pets", the RSPCA said. ( Flickr: Corinne Benavides )

In a statement on its Facebook page, Dogs Victoria said any dog had the capacity to be unpredictable.

"The circumstances surrounding this event are not clear, however the outcome is horrific.

"Dog attacks are shocking for all involved and we wish to offer our sincerest condolences to the family, friends and first responders affected by this tragedy," Dogs Victoria president Wayne Fleming said.

Tegan McPherson, the RSPCA's head of operations, said her organisation backed a "deed-not-breed" approach to judging animals.

"Breed alone isn't a reliable predictor of aggressive behaviour," she said.

"It can be influenced by genetics, learning experiences and training and its environment.

"You've got American Staffies that are wonderful family pets, but you also have ones hitting the news for the wrong reasons."

Ms McPherson said there were a number of reasons why a seemingly healthy dog could suddenly attack.

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

"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."

Melbourne lawyer Brett Melke, who says he has represented dog owners in more than 500 cases, said focusing on individual breeds was "misguided".

"In terms of dangerous dogs, I've had cocker spaniels, I've had golden retrievers, I've had jack russells and border collies," Mr Melke said.

He said it was vital that people registered their dogs.

"When dogs are unregistered there's no way to track what they're doing, there's no way to track where they are. It's a dangerous scenario," he said.