The boy was given first aid at the scene before being taken to Ashburton Hospital, where he was stabilised before being taken to Christchurch Hospital by the Westpac Rescue Helicopter.
People at the house said they were too upset to speak to the media.
The boy's injuries were originally believed to have been "very serious'', but late yesterday afternoon Ashburton Police senior sergeant Grant Russell said they were not as serious as first feared, although his condition was still listed as critical.
The dog was seized by Ashburton District Council animal control officers and impounded before being put down.
The council's regulatory manager Richard Wade said the dog was a family pet with no history of aggression. It had been registered since 2006, was microchipped and not come to the attention of dog control officials before.
"There is nothing about the dog that we are aware of that would have alerted us to the potential for an attack of this sort,' said Mr Wade.
"This is a real tragedy and our thoughts go out to the family,'' said Mr Wade.
The co-operation of the family had made the process straightforward.
"The action by the family is certainly appreciated as it enables us to address the issue quickly,'' Mr Wade said.
Such incidents were extremely rare in Ashburton, Mr Wade said.
In the last year the only other incident he'd dealt with was a dog that attacked some animals and then attacked the animal control officer sent to capture it. That dog was eventually destroyed following a court dispute with the owner.
Institute of Animal Control Officers Les Dalton says it doesn't matter what breed the dog is, parents need to be careful leaving their children alone around animals.
"To leave a young child with a dog alone, especially with food, children do taunt dogs and dogs react differently, and dogs that haven't been social around young children, particularly people visiting another dog, is always a very high risk,'' he told Newstalk ZB's Mike Hosking.
Yesterday's case is currently with the police so Mr Wade said it was too early to say whether the dog's owner would be prosecuted.
"We have 7000 dogs in the district and this is thankfully a very rare occurrence,'' said Mr Wade.
"People in the district are generally very conscientious pet owners and it makes an incident like this all the more shocking.''
Victim Support is assisting the family. Mr Russell said enquiries were continuing.
