Police probe love triangle theory in stabbing murders

A Melbourne man whose wife was found stabbed to death along with their grandson, may have been in a relationship with a younger woman.

Police found the 60-year-old victim in the backyard of their Albanvale home on Tuesday. Her 61-year-old husband was bound and gagged on the living room floor.

The couple had been babysitting their four-year-old grandson, from Maidstone, who was found dead in a bedroom of the home.

A woman, 45, is under arrest and being questioned by investigators.

Detective Senior Sergeant Stuart Bailey has told reporters they don't know the motive.

But one line of inquiry is the possibility of a romantic relationship between the younger woman and the grandfather.

"We think it's got something to do with a relationship between the 45-year-old lady and the 61-year-old man," Sgt Bailey said.

The grandparents had been babysitting while the boy's parents were at work, something they do every Tuesday, police said.

The younger woman had arrived at the home in the morning and the "horrific" incident occurred after the man's wife returned home.

"She's been fatally stabbed in the backyard," Sgt Bailey said.

Her grandson's body was found in a bedroom after his grandfather, who was tied up in the living room, asked how he was.

"The scene is quite horrific," Det Sen Sgt Bailey told reporters on Tuesday night.

The boy's parents were inconsolable and devastated, he said.

The grandfather is recovering from minor injuries in the Royal Melbourne Hospital.

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Neighbours spoke of hearing a dispute at the address before the bodies were found.

A woman, who wished to be known only as Nasma, said she had lived in the area for more than 16 years and was shocked when she heard a woman and child had been killed.

"It's scary," she told AAP.

"Something like that, it just doesn't make you feel safe."

No charges have been laid.

Morning news break – April 1