A man is dead after the tractor he was travelling in rolled, just south of New Plymouth.
It is believed the tractor rolled onto the driver after it collided with a car that was overtaking.
Emergency services were called to the scene, on South Rd in Oakura, at 4.23pm by a member of the public.
Fire crews are working to extricate the body from the tractor.
A witness said a crane was being used to lift the tractor.
A woman who lived nearby said she heard a loud crash and ran outside to see what had happened.
"An ambulance is still there. We think another car was overtaking and hit the tractor, which flipped then landed on the driver - he was an old guy," she said.
"Paramedics worked on him for ages and ages - it was really sad."
The witness said a woman and her little girl were travelling behind the tractor, which was heading north, when the incident occurred.
The car that collided with the tractor is understood to have been travelling south - and the crash occurred just after the start of a 100km/h zone.
A police spokeswoman said they were called to the scene at 4.24pm.
"Initial reports suggest the tractor was clipped by a car and the tractor then flipped, blocking both lanes - this will be confirmed once an investigation is underway."
The Serious Crash Unit has been informed of the accident, and diversions are in place.
According to NZTA, South Rd - State Highway 45 - is closed near Wairau Rd, and motorists are advised to follow directions of emergency services.
The witness said there were a number of crashes on the same stretch of road, and she had asked NZTA many times to move the beginning of the 100km/h zone further south.