State Highway 11 in the Bay of Islands will be shut for at least two weeks after a huge slip came down during heavy rain on Tuesday.
The 80m-long slip is blocking the road just north of Kawakawa, in the Lemons Hill area.
The NZ Transport Agency said motorists in the Far North would need to take a detour via SH10, adding an extra 10-20 minutes between Paihia and Kawakawa.
Engineers estimate it could take at least a fortnight to make the hillside safe enough to reopen the road, NZTA system manager Steve Mutton said.
"A lot depends on the weather. It's fine right now, but there have been days of heavy rain and more is forecast next week. More rain could potentially see the slip start to move again.
"We have started initial work on the slip and we must ensure the safety of our crews. Our engineers will be using a drone to help survey the extent of the slip."
The slip came down during a night of thunder and downpours which left roads flooded and forced 16 schools to close.
All other state highways in Northland are now open, and crews are checking roads for damage caused by the flooding.