A neighbour told the Herald Sun he tried to spray the dog with water to distract it after he heard screams.
"There was no stopping it. It's a giant pit bull," he said.
There are reports the deceased man had been recovering from an operation and was in a motorised wheelchair.
A neighbour told The Age that she would frequently chat with the victim at the supermarket.
"He's been a very lovely person. We will miss him a lot, especially my husband. We love him."
A woman in her 60s was also injured in the mauling, police said. She was taken to the Royal Melbourne hospital for treatment where she is thought to be in a stable condition with severe injuries to her upper body.
Efforts to capture the dog proved futile until a ranger eventually subdued it at around 10.30pm. It is being held by Whittlesea council.
Senior Sergeant Glenn Parker said the dead man's daughter-in-law gave consent to destroy the animal.
"The dog belongs to the occupant's son, the dog's been here for quite some time," he told reporters at the scene.
"He (the dog) will be taken to a veterinarian, and my understanding is that the owner of the dog has actually consented to that dog being put down," Parker said.