A warning has been issued for water users after a truck lost several containers resulting in a chemical spill on State Highway 29 this morning.
The highway was closed for most of the day after the incident about 500m south of the Kaimai Range summit.
Waka Kotahi New Zealand Transport announced about 6pm it had reopened but asked drivers to take care in the area.
Waikato Regional Council said on its Facebook page that a Ph test had been carried out on Mahina-a-rangi Stream.
"Although the water gave a neutral reading, we ask the public to refrain from any water takes at the moment. We intend to visit this location again as well as other locations downstream of the incident site for further monitoring of the various waterways.
"At this stage we can't rule out the possibility that the contaminant has been caught in pockets, which could then flow downstream if there's rain. We've checked the weather forecast, and some rain is forecast to occur overnight."
The council earlier issued a statement after the incident warning the chemical may have entered a nearby stream.
"The truck was carrying large containers filled with sodium hydroxide, with approximately 3000 litres spilling from the truck, resulting in the closure of the Kaimai Ranges," the statement said.
"The council's incident response team is attending the scene to assess the environmental impact of the incident."
The council said until more was known about the extent of the spill water users were urged to exercise caution.
The road was expected to remain closed for much of the day to enable clean-up and motorists are encouraged to delay their travel where possible.
Detours were in place through State Highway 36 and State Highway 5 via Rotorua or State Highway 27, State Highway 26 and State Highway 2 through the Karangahake Gorge and on to Tauranga.
These detours added considerable time to journeys and are not suitable for High Productivity Motor Vehicles (HPMV)
Several containers fell off the truck which caused significant delays this morning.
The incident was reported at 6.15am.
A Fire and Emergency NZ spokeswoman said four fire appliances and one water tanker were sent to the scene.