A Fenz spokeswoman said the crash happened about 6.40pm this evening, between Lawrence and Waitahuna.
Crews from Lawrence, Waitahuna and Milton responded initially, and "a support vehicle" was sent from Dunedin with "some extra absorbent material" to help clean up the spills.
The spills were believed to be a mixture of fuel and hydraulic oil, and the Dunedin unit was sent to ensure it did not leak into nearby waterways.
A police spokeswoman said the excavator was down a bank, and the truck and trailer were partially blocking SH8, and the road is now closed between Clarks Flat Rd and Waitahuna West Rd.
Road users are asked to avoid the area or to follow a detour.
There were no reports of injuries, she said.
There were no reports of injuries and the cause of the crash was not yet known, she said.
A detour is available for southbound traffic by turning right onto Clarks Flat Road, then left onto Waitahuna West Road, and back onto SH8.
The reverse applies for northbound traffic.
The detour is not suitable for heavy vehicles.
