A man is in critical condition after being ejected from his small truck when it hit a car in Wellsford.

The incident happened on State Highway 1, near Wayby Valley Rd, just after 4pm.

Auckland Rescue Helicopter co-pilot Aaron Knight said the man in his 20s suffered head trauma, a fractured leg and collarbone.

"The occupant of the other vehicle was relatively unscathed.

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"The weather was pretty treacherous in the area."

Knight said the man was treated at the scene, including being intubated, before he was airlifted to Auckland Hospital.

A St John spokeswoman said earlier this evening that an ambulance, helicopter and doctor were at the scene.

Diversions are in place at State Highway 1 and Kaipara Flats Rd and again at the intersection of State Highway 1 and 16.