The additional money for the project provides for the construction of a dual-lane roundabout, a new two-lane bridge over Otaika Stream, two southbound lanes from the roundabout with safe transition back to a single lane south of the new bridge and the installation of a central median barrier.
The additional work has an estimated cost of $23 million and will extend the current construction programme into early 2022.
Enabling works will continue and the detailed design of the new scope is expected to be completed by September 2020.
Associate Transport Minister Shane Jones said the extra funding was a "Christmas windfall" and when completed would make a huge difference for those driving the road on a daily basis.
He said improving the road between Whangārei and Marsden Point was a priority and he would keep pushing for more money to be spent on the dangerous stretch of highway.
The section of SH1 between Whangārei and Marsden Point carries about 19,000 vehicles per day, and of that 13 per cent is heavy freight traffic, transporting a significant portion of the forestry traffic in Northland.
As part of the project, work is being done on the Portland intersection as well.
Earthworks at the Portland Rd intersection are expected to take about two months to prepare for widening of the road to accommodate the left-turn slip lane.
That left slip lane will allow southbound traffic to separate from SH1 as they slow to enter Portland Rd.
As part of the widening at Portland Rd a right-turn bay for northbound traffic entering Portland Rd will also be installed.
This widened intersection, including the left-slip lane will also improve safety for traffic turning right out of Portland Rd and increased driver visibility.
Portland Bridge widening works have been under way over the last few months to accommodate the widened intersection.
