The Desert Road is closed after a crash involving two cars and two truck and trailer units left a young child dead, and four other people in hospital.
Senior Sergeant Fane Troy from Bay of Plenty Road Policing said three people were taken to Waikato Hospital by helicopter in a critical condition.
Another person with moderate injuries was taken by ambulance to Whanganui Hospital.
Mr Troy said the gender and age of the child who died, and that of the injured, would not be available until all next of kin were informed.
He said it would take some time to contact them as some family members were overseas.
The Serious Crash Unit and the Commercial Vehicle Safety Team are investigating the crash, in the south-bound lane of State Highway 1 north of Waiouru, between Waipakihi Rd and Rangipo Intake Rd, just before 3.30pm.
The Transport Agency urged motorists to take the National Park route through SH46 and SH49, until the road re-opened. It is a 90-minute detour.
The death brings the Easter road toll to two, after another person died in the morning in a crash on Tirohanga Road in Bay of Plenty.
In the South Island, motorists travelling through Arthur's Pass are being warned to expect delays, due to traffic congestion and a fault with local traffic signals.
Westbound traffic on State Highway 73, was particularly congested, and holidaymakers should allow for extra time on their journeys, a Transport Agency spokesperson said.
Further south, motorists are being warned to expect delays on State Highway 1 heading south into Rakaia.
Motorists are being told to be patient due to large numbers of vehicles on the roads.
