A section of State Highway 1 at Moerewa's Turntable Hill damaged in last month's storm could take up to six weeks to fix.
The highway is still open to traffic in both directions but the passing lane and one shoulder have been closed since the storm.
The underslip, just downhill from Hautapu Rd, is about three metres in length, with a drop of about 2m.
The slope has destabilised for another 5m below the visible slip.
A New Zealand Transport Agency spokesman said design work to repair the underslip was in progress, with repairs expected to be complete in four to six weeks.