Witnesses said the quad bike rolled a couple of times before coming to rest. The woman was thrown free of the machine.
Senior lifeguards checked that she was conscious and breathing, Mr McKernan said.
The Northland Electricity Rescue Helicopter was called at 11.30am and pilot Pete Turnball said the girl was flown to Whangarei Hospital in a serious condition. A police serious crash investigator was called and will examine the quad bike.
Meanwhile, a driver trying to avoid a collision ended up writing his car off when it rolled into a ditch.
The crash happened on State Highway One at Mata, north of the Marsden Point roundabout about midday yesterday.
The car had four people on board when the driver said he took evasive action after another vehicle overtook and cut him off, Sergeant Lance Goulsbro of Northland Police Highway Patrol said. The car rolled and ended up on it's roof in a ditch.
"It's fairly well written off," Mr Goulsbro said.
Fortunately, the driver and his three passengers managed to walk away unharmed.
However, the incident at Mata caused traffic hold-ups for travellers while police and fire units cleared the scene and waited for the car to be removed from the scene.
Overall the State Highway network had been busy, but free of serious crashes Mr Goulsbro said.
"Traffic has been steady but everyone's been quite well behaved," he said.
"There's been a number of tickets given out for people breaking the 4km/h speed limit [tolerance] but nothing serious."
Traffic was steady all through Saturday, a little lighter on Sunday, and was expected to be steady today.
Mr Goulsbro wanted to tell motorists to keep up the good driving habits he had seen over the weekend on the State Highway network.
"Remember to be patient. It's better to get home a little later than not get home at all," he said.