The incident came on the eve of the holiday season, with the official Christmas road toll period due to begin at 4pm today.
The national road toll stood at 285 last night.
"This is a tragedy - a catastrophic scene - and there will be a number of people who will be affected by this incident," said Northland's road policing manager, Inspector Murray Hodson.
The Range Rover overturned on the other side of the road. The truck's trailer and its logs bound for Marsden Pt remained upright, but the truck and cab unit was left jack-knifed across one lane.
Two bags bearing the logo "North Shore Aquatic Centre" were visible in the rear of the mangled car.
Two men and two girls in the car were killed, while the 8-year-old boy was airlifted with critical injuries to Whangarei Hospital, where family members were with him.
He was flown to Auckland's Starship Hospital last night, where he was in a stable condition this morning.
The truck driver was last night being treated in hospital for cuts and grazes suffered as he slid along the road in his overturned cab for dozens of metres after the collision, towards the end of what was to have been his last delivery for the year.
He had to be cut from his cab by firefighters. Smith and Davies trucking company Northland general manager Daron Turner said his highly experienced logging driver, aged about 60, told him he was powerless to prevent the tragedy. "They were on the left-hand side of the road and did a u-turn across in front of him, and now they're not here any more.
"The car rolled the truck over."
Mr Turner said the truck was in the left-hand lane of the highway, and the driver was looking in his rear vision mirror to check for any traffic overtaking in a passing lane before he looked back in front to see the car right "on him".
"He was scanning his mirrors as you normally do, and he turned back and this Range Rover was across the front of his truck and it was all over.
"He didn't have time to do anything, mate - next thing he's sliding down the road on his side."
Mr Turner, who rushed to the crash scene and later visited the driver in hospital, said the man was "just traumatised" by the accident.
"He's going to be haunted for a long time," he said of the driver, who he described as one of the best of his 100-strong Northland workforce and a well-respected Maori community member. "He's putting on a brave face but it's upset all of us - we've all been very traumatised.
"He's a very nice guy, it's just a very sad thing to happen to him. But there is nothing you can do to train a driver to avoid accidents like that.
"He needs to be able to settle down, with his wife and family. Right now he's recovering from a tragedy."
Mr Turner made a plea to other road users: "Please take your time, please drive safely, be very careful on the road this summer."
He had emphasised to the driver that the police acknowledged there was nothing he could have done to avoid the crash, which was "small consolation for him" and the company would be recommending that he accept its offer of professional counselling.
The company's Auckland-based managing director, Graham MacKinnon, said GPS monitoring confirmed the truck was travelling no faster than the 90km/h speed limit for heavy vehicles.
He said the company had held a pre-holiday safety meeting with all its drivers late last month "to make everyone aware they would be sharing the road with people who are probably distracted and not in their familiar areas".
Nearby resident Sandra Wrack said she was on her veranda in Uretiti Rd about 250m away when she heard "a boom followed by another one, then deathly silence".
She was terrified for the safety of her son and grandson, who were on their way back from a visit to Whangarei, until they phoned to say they were eight cars back from the crash.
Mrs Wrack wondered if the car's occupants, who were of Asian descent, had been heading to a crab-gathering spot popular with Auckland's Asian community, but had missed the Uretiti Rd turnoff before deciding to turn back.
- Additional reporting: NZME and Northern Advocate
