The section of highway closed at Thames has now reopened, says Waka Kotahi NZ Transport Agency.
State Highway 25 between Mackay St and Maramahi Rd in Thames was closed for hours while emergency services responded to a crash involving a car and a motorcycle at the intersection with Banks St.
Northbound traffic on State Highway 25 was being turned back at Kopu and southbound traffic was being turned back in Thames, police said.
Police were notified of the accident at 11.55am.
One person was taken to Waikato Hospital in a critical condition, police said.
Motorists had been advised to avoid the area and expect delays.
The Serious Crash Unit was in attendance.
Stay up to date with the Waikato Herald
Get the latest Waikato headlines straight to your inbox Monday to Saturday. Register for free today - click here and choose Local News.