The "harrowing" crash happened about 1pm on Saturday in Hawke's Bay.
Police said a police patrol on the outskirts of Waipawa recorded a vehicle heading into town at 130kmh.
The patrol turned to follow the vehicle and shortly afterward, saw it had smashed head-on into another vehicle on High Street (State Highway 2).
Three people were taken to Wellington Hospital - two in a critical condition.
Police said they remained in a critical condition on Sunday afternoon.
The third person taken to Wellington Hospital had minor injuries and had since been discharged.
A fourth person was taken to Hawke's Bay Hospital, where they remained in a serious condition on Sunday.
A dog was in the vehicle and sustained minor injuries, police said.
It was taken to a local vet to be checked and had since been discharged to its owners.
Police said it was believed that excessive speed was a factor in the crash.
A 19-year-old man was arrested at the scene and charges were being considered.
"Incidents like this are harrowing for all involved, including the victims, their families, members of the public who witnessed the crash, and first responders," Hawke's Bay area commander Lincoln Sycamore said.
"We are providing support to those affected and our investigation into the circumstances of the crash is ongoing."
