A milk tanker driver had to be cut free of his cab this morning after the b-train he was driving rolled at about 5.30am.
The 40 tonne tanker was fully laden with lactose when it lost control 5km north of Mokau in Awakino Gorge.
Two fire trucks, from Mokau and Urenui, attended the scene on State Highway 3.
Central fire services shift manager Mike Wanoa said the driver was extricated from the cab just before 6.30am and is understood to be on his feet and moving around.
He was being assessed by ambulance staff and an emergency helicopter was inbound.
Mr Wanoa said the tanker careered off the road and was hanging into the river area.
The road is now open but two cranes will be required to lift the vehicle back onto the road.
The tanker's load will be drained before the wreckage is removed.
Waikato Regional Council emergency response manager Rob Dragten said milk and other similar dairy product was toxic to fish and other aquatic life.
The tanker is believed to have been carrying around 25,000 litres of dairy product and material is reported to be leaking towards the Awakino River.
He said council staff would make an environmental risk assessment at the scene to see how to manage the environmental threat.
"We will work closely with emergency services to look at what we can do to prevent environmental damage in the wake of this accident,' said Mr Dragten.