Road users were unable to access the Whitianga township from the Tairua side of the Coromandel Peninsula.
SH25A across the peninsular is also closed for maintenance work.
People needing to travel to Whitianga must take the significantly longer route via Coromandel township until police re-open the road, the agency said.
The detour would add 90 minutes to travel times.
The Serious Crash Unit have been notified.