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The newborn, who had yet to be given a name, died with parents Lisa Crowley, 23, and Richard Melling. Ms Crowley's son Brody, 2, and Mr Melling's son Jordan, 11, were also killed.
Ms Crowley was driving and the baby was in a capsule in the back of the car.
They were last seen at their home in Whakamaru at 8pm on Sunday. It is not clear where they went and when they crashed.
Post-mortem examinations are to be carried out to determine whether they died instantly or while trapped in the car.
Food from McDonald's was visible on the front seat of the wreck, but police had not discovered last night where the family had travelled to after leaving their home on Sunday.
"Their vehicle appears to have been heading towards Whakamaru, suggesting that at some stage they had left the settlement to an unknown destination and were then returning," said police spokeswoman Kim Perks.
The force of the impact wrapped the car around a tree about 20m from the road.
The wreckage was found less than an hour after emergency services converged on a crash that killed two Indian nationals on nearby State Highway 1. They have not been named because police are still contacting their families overseas.
About 1.05pm a car travelling south towards Taupo and a truck collided head-on. Both occupants of the car, two Indian males visiting New Zealand, died at the scene.
The male truck driver and a female passenger, both from Kaitaia, were treated for minor injuries at Tokoroa Hospital.
As emergency services were attending that accident, two contractors assessing a forest area on SH32 near Whakamaru found the car with the Melling and Crowley family inside.
The car is understood to have crossed the centreline before going down a bank and hitting the tree.
Police had to wait for the Fire Service to finish at the other crash before the bodies and wreckage could be recovered.
A man who lived on the same street as the couple said he heard them leaving the house on Sunday.
"They went out very early on Sunday morning. I don't know if it was a holiday or what it was. They left the lights on and everything."
He said he had spoken to police but would not comment further. Other neighbours refused to speak about the couple or the crash.
"This is an absolute tragedy; a Christmas in ruins for a number of families and friends. Our hearts go out to them," said police area commander for Taupo Inspector Steve Bullock.
"It is a timely reminder of just how easily lives can be changed in an instant. I would ask that everyone please reflect on what has happened today and do everything possible to ensure that we do not see more lives lost on our roads over the holidays.
"We don't yet know what caused these particular crashes but we do know that speed, alcohol, fatigue, inattention and a lack of restraints are major factors in deaths on our roads. Please drive to the conditions and to your ability at all times."
- Nikki Preston, New Zealand Herald