Hawke's Bay Road Policing Sergeant Ray Kirkby said the car appeared to have clipped a truck which was also travelling toward Napier, and was "flicked" into the oncoming lane where it collided with the southbound truck.
It is understood the couple, both in their 50s, in the truck had been on their way to work in Hastings.
The driver was treated at the scene while the woman was taken to Hawke's Bay Hospital where she was later said to be stable.
The woman driver of the car, in her 20s, was treated and discharged.
North and south-bound traffic was backed up nearly 2km in both directions as emergency crews worked to clear the scene.
Traffic was diverted through Taradale and Brookfields.
"There was debris everywhere," Mr Kirkby said, adding the expressway was re-opened about 9.20am.
Within minutes of the crash there was a second "nose-to-tail incident involving three cars on the approach stretch to the overbridge as traffic backed up".
"People simply have to drive to the conditions, stay alert and be prepared to stop at any time."
The main crash was still being investigated and no charges had been laid at this stage.
Police were also called to the corner of St Aubyn and Miller Sts in Hastings about 11.30am yesterday after a two-car collision.
One driver received minor injuries while the other was taken to hospital to be assessed for moderate injuries.
Emergency services were busy again in the afternoon working at a two-car crash near Pukehou at the intersection of State Highway 2 and Te Onepu Rd, 27km south of Hastings.
Police Senior Constable Tim Rowe said a woman driving south had pulled up to the intersection and was waiting to turn right when her car was hit from behind by a campervan.
"The gentleman in the van had been momentarily distracted by checking his rear vision mirror and when he looked forward realised the vehicle in front of him was stationary."
The van attempted to swerve but hit the rear left-hand side of the car, shunting it across the road into the path of a BMW car, missing it.