Speed limits on parts of the alternative road between Picton and Christchurch are to be lowered as a safety precaution.
With State Highway 1 still not fully open following last month's earthquake, the volume of vehicles travelling the alternative inland route has quadrupled.
The NZ Transport Agency says road safety engineers have reviewed the route and speed limits have been reduced along parts of state Highways 6, 7, 63 and 65.
Daily traffic on State Highway 63 through the Wairau Valley - between Blenheim and St Arnaud in the Nelson lakes area - has grown from fewer than 400 vehicles a day to more than 1200, according to the Transport Agency.
Highway manager Frank Porter said reductions of up to 20km/h have been made on sections where the road was windy and narrow, as well as at key intersections and through the townships and villages along the route.
He said the changes would add very little to drivers' journey times.
State Highway 1 to the south of Kaikōura re-opened yesterday for the first time since the 14 November 7.8 magnitude earthquake.