State Highway 60 will be closed for several hours following a chemical spill on the Tākaka Hill Highway in the Tasman area.
Seven-hundred litres of sodium hydroxide, otherwise known as caustic soda, is leaking from a truck on to the road. It is still unclear as to what caused the leak.
The chemical has spread to nearby paddocks and farmland as well as several hundred metres along the roadside.
St John sent two ambulances to the scene, taking one person in a moderate condition to hospital.
Nearby staff from Fonterra Tākaka are assisting Fire and Emergency NZ with the clean-up.
Police are advising motorists to expect delays.