The company that owns the bus which crashed near Shannon in Horowhenua on Sunday night says initial reports indicate it tried to avoid an oncoming vehicle, causing it to clip a bridge then roll.
The accident happened just after 6pm on State Highway 57.
Tranzit Coachlines says the bus, which was carrying 42 people, rolled and the passengers were trapped.
Seven people were taken to Palmerston North hospital with moderate injuries.
Another 21 were taken to the Club Hotel in Shannon for assessments by ambulance staff.
Tranzit Coachlines had hired the bus to another company, Intercity.
Tranzit general manager James Snelgrove says the driver, who works for Intercity, reported the bus pulled left to miss an oncoming car and clipped the bridge, skidding along the wet verge then rolling.
The driver suffered minor injuries.
Janelle Bishop, the daughter of the Club Hotel's publican, says she went to the accident scene and saw the bus lying upside down.
She says the passengers were badly shaken and a lot of them could not remember what happened.
Second bus crash
The crash was the second in the area involving buses within two-and-a-half hours on Sunday.
Just after 4pm a bus carrying 30 people and a van with six people onboard collided on State Highway 1 at Riverbank Road, near Otaki.
Dave Greenberg of the Life Flight rescue helicopter says the bus driver appears to have had a medical episode, which led to the collision.
He was flown to Wellington Hospital. No other injuries have been reported.
