A 61-year-old Mount Maunganui motorcyclist suffered serious arm, shoulder and torso injuries, as well as a possible fractured pelvis.
The man remains in a stable condition in Tauranga Hospital's intensive care unit.
At 7pm, a vehicle travelling north on State Highway 2 at Aongatete left the road and rolled through an area of scrub before coming to rest upside down in a drain.
Police said the driver lost control on a moderate right-hand bend, struck a road sign and narrowly missed a concrete power pole.
Early indications suggested the driver had been speeding.
Katikati volunteer fire chief Joe Manukau said the driver appeared to have lost traction on the road and ended up on an "awkward angle" off the road.
A 48-year-old Katikati man was taken to Tauranga Hospital with moderate injuries before being treated and discharged.