The worst of Sunday evening's crashes occurred on State Highway 1, about 15km south of Kaitaia, around 9.15pm.
Senior Constable Warren Bunn, of the police Serious Crash Unit, said the vehicle was heading towards Kaitaia on the northern side of Mangamuka Gorge when it left the road after a slight bend and ended up in rock-filled drainage ditch.
It travelled about 100m in the ditch before rolling and coming to rest on its side against a stand of bamboo, just missing an abutment of Thompson's Bridge.
The cause of the crash was not yet known but a number of occupants were not wearing seatbelts, including the deceased. Most were locals and aged under 20.
Northland's top roading cop, Senior Sergeant Steve Dickson, said if everyone had been wearing seatbelts it was likely the injuries would have been far less severe.
He believed three passengers weren't restrained and had been ejected in the crash.
''It was a very avoidable death. And when you throw into the mix that most of the country is at alert level 3, they should have been at home. That would have also prevented the crash.''
Dickson said the fatality was ''a very sombre reminder'' about the need to always wear a seatbelt and drive to the conditions.
Kaitaia deputy fire chief Ross Beddows said with five people in the car it was a chaotic scene when emergency services arrived.
A passing motorist and a local resident were tending to the injured who included a woman lying on the road.
One man had made his way to a nearby house while a woman was trapped in the back seat with serious injuries including a broken leg and lacerated arm.
A male passenger had been thrown from the car and was found below the bridge in a critical condition. He died before he got to hospital.
The initial ambulance was delayed because staff were still tied up with the aftermath of an earlier crash on Fairburn Rd.
Beddows said it took half an hour to cut the back seat passenger out of the wreckage.
It was a difficult rescue because the car was on its side, down a bank and leaning against bamboo. Firefighters got her out by cutting off the car roof.
She was in a serious but not life-threatening condition.
Beddows said the volunteers performed well in challenging circumstances. He also thanked passing motorists and local residents who helped.
He had a simple message for Northlanders: ''Just put your bloody seatbelt on. It's difficult to get ejected from a car if you're belted in.''
A St John spokeswoman said two ambulances, one Northland Rescue Helicopter and one Westpac Rescue Helicopter responded.
Two patients were flown to Whangārei Hospital in a serious condition and one was taken to Kaitaia by road with moderate injuries. One patient was treated at the scene.
In the earlier crash, about 8pm, a ute travelling on Fairburn Rd, east of Kaitaia, rolled after a taking a corner.
The driver, who was not wearing a seatbelt, was thrown from the vehicle.
''He was very fortunate the ute didn't roll onto him,'' Beddows said.
A child in the front passenger seat, who was restrained, was shaken but not injured.
Firefighters set up a landing zone for the rescue helicopter but the driver was instead taken to Kaitaia Hospital by ambulance then flown to Whangārei in a serious condition.
Locals helped tip the ute back onto its wheels and off the road so traffic could get past.
Dickson said in that case a 49-year-old man was taking his son home when a small error caused his vehicle to leave the road.
Northland's first lockdown road death was that of Tristan Kearney, 43, who crashed on Peria Valley Rd on August 23. His vehicle was found by a farmer the next morning. He was not wearing a seatbelt.
According to the Ministry of Transport 23 people have died on Northland roads so far this year, more than all of 2020 or 2019, and on par with the full-year totals for 2018 and 2017.
Sunday's crashes capped an extraordinarily busy day for Kaitaia's volunteer firefighters with five call-outs between noon and 9.15pm.
The other incidents included a permitted fire in Victoria Valley Rd and a false alarm triggered by a smoke detector in a North Rd flat.
