A water tanker has rolled on the Waikato Expressway at Hampton Downs spilling its contents over the road.

The incident happened about 5.40pm.

St John spokesman James Ihimaera said two people suffered serious injuries in the crash and have been taken to Waikato Hospital.

Emergency Services are at the scene on the southbound lanes and banked up traffic is still squeezing through one lane.

Police northern communications Inspector Willie Taylor said the tanker driver was injured.

It's understood two people received serious injuries in the crash.

Mr Taylor said the cause of the crash was not yet known.

‘‘It’s going to take a while to get it off the road and get it open.’’

Four fire trucks were called to clean up the mess but have now left the scene.

Northbound lanes are open.

- © Fairfax NZ News