A milk tanker crashed near Mataura in Southland just before 3am on Saturday, spilling 12,000 - 15,000 litres of milk and bringing down several powerlines.
Police say the tanker went off State Highway 93, but the driver got out unharmed.
The milk went into the Mataura River, but was well diluted as the river is in flood.
Diversions are in place for light traffic only, on Doctors Road and Boundary Road, but large trucks are advised to divert onto State Highway One or to be prepared to wait.
The Fire Service says another Fonterra tanker is at the scene, removing milk from the crashed vehicle.
Powernet says most residents in Mataura should have their power supply restored by now.
Southland Regional Council says there is likely to be no lasting environmental impact from the spillage.