A 2-year-old boy was killed after being attacked by a mastiff cross last Sunday afternoon in Deniliquin in southwestern NSW.

According to police investigation, the boy was staying at a relative's home when he was attacked by the dog at about 2:30 p.m. (AEST).

His 70-year-old grandmother tried to stop the dog from mauling him when his mother arrived shortly and managed to fight the dog off.

The toddler and his grandmother were rushed to the Deniliquin Hospital by NSW Ambulance Paramedics, but the boy died a short time later. His grandmother was treated for cuts, bruises and exhaustion.

The mastiff cross, which was not considered as dangerous, was already put down by the authorities and council rangers.

"Dangerous dogs must be muzzled whenever they're outside their owner's property, which must have sufficient "containment measures,” said Des Bliske, the Deniliquin council manager.

"People that own dogs generally control them and keep them very well contained,” he added.

"We've probably had three or four (attacks) over the last 12 months, not all of them involving people.”

The detectives were now investigating the death and preparing a report for the New South Wales Coroner. It’s unlikely that charges will be laid against the dog owner.

Detective Inspector Darren Cloak said that the family and local residents were “shocked” and “distraught” by the tragedy.

Mastiff dogs are known as noble giants. However, some tended to be aggressive when they get excited.

A similar incident happened back in 2011 in Melbourne when a pitbull terrier cross attacked and killed a 4-year-old girl.

Relatives of the girl questioned the dog's owners for not helping save the victim. "They didn't try to help her," the girl's uncle said. According to The Australian, pitbulls are a restricted breed in Victoria.

Colin Muir, president of the American Pit Bull Terrier Club of Australia, pointed out that the problem was not a particular breed, but the owner's irresponsibility of caring for their dogs.

"A breed doesn't do these things, an individual dog does, and it's irrelevant what the breed is," Mr Muir said. "Is a pitbull more dangerous than a chihuahua? I would say yes, but it's all relative," he said.

In the past 7 years, reports have shown that three children had already been killed by dog attacks in NSW and one in Western Australia.