The attack comes less than a fortnight after a 6-year-old boy was mauled by a dog in Paengaroa.
Otumoetai woman Kelly Fisher said her son had been in Maketu with friends for a birthday party.
The boys were playing in the local skatepark and saw the "medium-sized, golden, short haired, maybe a mastiff cross" dog nipping at other skaters, she said.
Mrs Fisher said the dog ran up behind Mitchell before he entered the skate bowl on his board and bit his leg.
"He never even saw it coming. It came from behind him. A couple of the puncture wounds were quite deep, it wasn't big and gory but they were decent bites," she said.
A group of teens seen with the dog called it over and took off, Mrs Fisher said.
Mrs Fisher said her son was shocked. Mitchell was a "stocky, hardy kid" but she questioned what could have happened if he had been smaller.
Mrs Fisher wondered if the dog's owner was aware of what it was doing to people at the park.
"If they don't care, something needs to be done. If they are responsible dog owners they will realise the dog cannot be out un-restrained. It comes down to the dog owners," she said.
The Western Bay of Plenty District Council compliance and monitoring manager Alison Curtis said Mitchell's family notified the council of the attack and an investigation was ongoing.
"The attack is of a significant nature, and council's goal is to ensure that the public safety is ensured," Ms Curtis said.
"All dogs should be under control at all times. This incident is a reflection of a dog not being adequately controlled."
It was too early to say if any prosecutions would be made, she said.
There had been 13 reported attacks in the district in the past three months and 56 in the past two financial years. That included a 3-year-old boy treated at Waikato Hospital for a facial injury after being attacked by a dog in Te Puke on July 6.
Tauranga City Council animal services team leader Brent Lincoln said there were 69 attacks on people in the last financial year. So far this year there had been 18.
- Additional reporting by Sandra Conchie