The mastiff cross attacked the two-year-old while he was at his grandmother's home in Deniliquin, New South Wales yesterday.
The incident occurred in the afternoon when the boy went outside to get an ice cream from the back fridge and the family pet followed him, The Australian newspaper reported.
It was not clear what sparked the attack.
The child's grandmother, aged 70, intervened but was unable to release the child. The child's mother arrived and fought the dog off, the paper said.
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The dog was seized by the Council rangers.
An Ambulance service spokeswoman said the boy suffered serious head and facial injuries and was transported to Deniliquin Hospital in a critical condition but died a short time later.
"They were really quite horrible head injuries that he received, including some serious scalp damage," she said.