If you were driving a late-model white sedan and witnessed Monday's school bus crash on Kauaeranga Valley Rd please get in touch with police.

Senior sergeant Graham Shields says the driver may hold vital information which could help their investigation into what caused the crash that occurred at about 10.45am.

The school bus carrying 50 Howick College students which crashed into a ditch on Kauaeranga Valley Rd. Photo: Supplied

'We are still looking to identify that white late-model sedan which was travelling in the opposite direction at the time of the crash,” says Graham.

'Our reason behind this is simply because that car was very nearby at the time of the crash and we would like to speak with that person to help see if they can assist us.”

Fifty students from Auckland's Howick College were on board the bus at the time when it left the road and crashed into a ditch 300m past the Thames Racecourse on Kauaeranga Valley Rd.

St John triaged 46 people on site in a first aid ten erected at the racecourse, of which 13 were transported to Thames Hospital with serious injuries. They have since been discharged.

Graham says the bus was recovered from the ditch last night and has since been sent away for inspection.

'It's gone away to be examined to see if there if there's anything mechanical which may have attributed to the accident.

'We have also interviewed the bus driver but we really would like to speak with the driver of the white sedan.”

If you were driving the late-model white sedan seen travelling towards Thames on Kauaeranga Valley Rd yesterday at about 10.45am please call the Thames Police Station on 07-867-9600