National Highway 29 on the Waikato side of the Kaimai Range is closed due to an accident.
It happened between State Highway 28 and Poripori Road on Tuesday afternoon.
Emergency services are on scene and motorists are being warned to prepare for long delays.
Detours are available.
Waka Kotahi, the NZ Transport Agency, said traffic from Tauranga to the north should use SH2 via the Karangahake Gorge.
From Matamata, use SH24, SH28, SH5 or SH36.
Waka Kotahi said motorists should consider delaying their trip.