The Transport Agency says unstable land may force a change of plans for part of a link between Auckland and Northland.
The Puhoi - Wellsford four-lane stretch of state highway is considered to be a road of national significance. It will continue the State Highway I motorway northwards from the end of the northern gateway toll road.
NZTA says engineers are investigating whether an upgraded route through the Dome Valley is feasible as the land is more unstable than first thought.
Northern highways manager Tommy Parker says he does not know if this will cause any delays and the authority is assessing all its options before choosing the best route.
He says the team is on track to start work in 2014 on the first stage of the project, from Puhoi to Warkworth.