Police say initial indications showed the south bound truck had crossed the centreline and collided with a northbound car.
They were now appealing for witnesses who might have seen both vehicles on the stretch of Waikato highway at 6.00am to come forward.
They were especially keen to hear from the driver of a white utility who was seen following the truck.
Fire crews had to use cutting equipment to free injured people trapped inside the wrecked vehicle.
The police serious crash unit was now investigating and road closures were still in place with diversions at Whakahoro and Manhire Roads.
'Bang makes herd jump'
Farm manager Danny Robinson was milking his herd of 200 plus cows this morning when he heard a "massive bang".
"I heard this bang and all the cows jumped although it's not unusual, sometimes they will knock a gate off its hinges. I jumped out of the (milking) pit to make sure it wasn't any cows doing something silly, but of course I wouldn't have seen any lights (because cows blocked view)."
Mr Robinson said it wasn't until a couple of minutes later when he heard trucks slowing down that he realised something serious had occurred outside the farm he works on.
"It was pitch black, it was only when I heard the trucks (slowing) because this road is just trucks all the time ... I went down to the end of the pit and could see all the lights."
He could see the injured were being tended to so he turned off his electric fence and waited about half an hour before heading to the scene.
It was then when he realised how bad it was as he saw that the car had no roof and there were still people trapped in the car.
It was the most horrible sight he had seen since his five years working on the farm and hadn't been able to stop thinking about it all morning.
Nightmare 24 hours on Waikato roads
It's been a nightmare 24 hours on Waikato roads with two serious crashes claiming lives of motorists.
Yesterday Hamilton man Kyle Sutherland died after the vehicle he was driving left the road and ended up submerged in a stream on the outskirts of Hamilton.
Waikato Road Policing Manager Inspector Freda Grace said prevention first remained the key message as the region's road toll claimed its 24th victim from 18 crashes.
"Police are asking drivers to really think about driving as a safety exercise, which is no different to any other activity that involves risk," she said.
"It is so important to ensure that you and your vehicle are fit for the journey. This is about ensuring we think 'prevention first' before we start the journey; is our vehicle in good condition, are we, as drivers in good condition - not fatigued, starting out tired, under the influence of alcohol or drugs, or distracted by other things going on".
Once on the road it was important to stick to the rules and always exercise patience and consideration for others, she said.
