An ambulance and a truck have collided near Levin, seriously injuring a paramedic and the truck driver, and blocking State Highway 1 in both directions.
Police said they were notified of the two-vehicle crash at about 10.45am.
Hato Hone St John mid central area operations manager Gareth Collings confirmed one of its rapid response vehicles was involved in the crash.
Two ambulances responded to the incident.
Both the paramedic in the rapid response unit and the driver of the other vehicle were treated for serious injuries it said, and were transported to Palmerston North hospital.
"The rapid response vehicle will be assessed today to determine whether it is road worthy," Collings said.
"Hato Hone St John takes the safety of the public and its people seriously and will be supporting Police with their investigation.
"Our thoughts are with those impacted by this incident and we are offering support to our people who were involved."
Police asked motorists to delay travel and if possible check the New Zealand Transport Agency Waka Kothahi Journey Planner for advice on alternative routes.
Geoff Lewis, who was passing through the area shortly after the crash occurred, told 1News the incident involved a gas bottle delivery truck and an ambulance.
Lewis said he understood the road was closed off due to the scene being a hazardous site.
NZTA said the serious crash unit was investigating.
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