The four other people suffered injuries ranging from minor to serious.Police, firefighters and paramedics are at the crash site, and two rescue helicopters were also called in.
The Palmerston North Rescue Helicopter flew two injured patients to Palmerston North Hospital - a seriously injured woman in her 30s and her baby.
The other two injured were taken to hospital either by the Wellington Rescue Helicopter or ambulance.
Diversions are in place around the crash through Mauriceville on Opaki-Kaiparoro Rd.
Earlier diversions were through a paddock, opened up by a "nice local", according to police.
The police serious crash unit is now investigating and police are warning motorists to expect delays.