Debris and a rolled truck and trailer are blocking State Highway 1 between Hamilton and Auckland during peak commuting hours.
The truck hit a barrier north of Te Kauwhata near Paddy Rd and continued over it before hitting an overbridge at 3.15am on Thursday, a police statement said.
Its occupants weren't injured, but the north- and south-bound lanes remained partially blocked in both directions early Thursday.
A crane has been called to remove the truck.
A lane in each direction was open for morning traffic, but both lanes will be blocked when the crane arrives to remove the truck and trailer.
State Highway 1 is the main highway for those travelling between Hamilton and Auckland.
Traffic is moving slowly, so motorists should avoid the area as there will be delays, the statement said.