Mr Coutts said if the dog had bitten the boy's throat he would have died.
Photographs of the boy's injuries supplied to The Daily Post were too graphic to publish.
It was not yet known if the dog got into the property through an open gate or if it jumped a fence.
The boy and his family do not want to be identified. Police are investigating and the dog's owner could be charged, Mr Coutts said.
The boy spent a night in Rotorua Hospital but is now back at home. Mr Coutts said he was left traumatised after the incident.
Mr Coutts said the dog thought to be involved was an american pitbull. He said american pitbull breeds made up 1.53 per cent of New Zealand's dog population but the dogs were responsible for 18 per cent of all attacks.
"They are totally misrepresented as far as statistics go."
He said between 50 and 60 people a year reported being attacked by dogs in Rotorua and the city followed the national average with about 18 to 20 per cent of those attacks being caused by pitbull breeds.
However, Mr Coutts said only about 20 per cent of all dog attacks were being reported.
"The other 80 per cent of the time the dog involved belongs to the family or a family friend and people are just not reporting it," he said.
"The people are being treated at hospital but that information is not being given to us."
Mr Coutts said there was no bylaw in Rotorua which meant pitbulls had to be spayed or neutered, but he warned that could change.
"In Rotorua we currently have 350 pitbulls which are not neutered or spayed."
He said dog bites often resulted in punctured skin with bite marks but this incident was much worse.
"It just shows what these dogs are capable of," Mr Coutts said.
Mr Coutts said he only started to see this sort of injury since pitbulls became a popular breed of dog.
"They seem to be popular with certain people."
His observations had shown the main reason dogs attacked people because of their environment.
"Ten per cent is because of the breed of the dog, 10 per cent is because of the dog's personality and 80 per cent is because of the environment the dog is in."
He said Rotorua dog owners needed to be more responsible for their pets.
"Every attack is preventable. If you have a dog it is your responsibility."