Fallen power lines have blocked State Highway 1 north of Kaiwaka, meaning it could be a long commute for Northland holidaymakers.
Police say a car came off SH1, south of Baldrock Road, shortly after 3pm on Thursday.
A boat being towed by the vehicle hit a power pole, causing the lines to fall across the road.
St John confirmed one patient was treated for minor injuries.
The highway has been closed between Mountain Road and Baldrock Road while emergency services work at the scene.
Police advise motorists that diversions are currently in place.
"Please expect significant delays in the area... avoid the area or delay your journey if possible," the New Zealand Transport Agency (NZTA) tweeted.
The incident follows NZTA's warning for motorists heading out of Auckland earlier on Thursday, with SH1 between Puhoi and Wellsford expected to have its worst traffic between 10am and 4:30pm.