The New Zealand Transport Agency (NZTA) will upgrade state highway 1 (SH1) and state highway 57 (SH57) as part of the Wellington Northern Corridor upgrade.

The SH1 and SH 57 upgrade project is aimed at increasing the capacity between Otaki and Levin, reducing congestion, improving traffic flow, enhancing the safety users and supporting economic development.

NZTA Wellington state highway manager Rod James said the agency looked at various options on different routes and decided that progressively upgrading the existing SH1 and SH57 would be the best approach.

"This will provide for early improvements to address current problems, while also providing for the staged development of a higher standard of highway as demand increases over time," James added.

The safety improvement extensions to the southern section of SH57 will not only provide an important connection between Wellington, Levin and Palmerston North but also ensure a safer transport and freight route where SH1 and SH57 connect.

The agency will design the first stage of the upgrade project and improve difficult areas on the route, such as the Forest lakes section, as well as rail and river bridges at Manakau and Ohau.

The project will also improve intersections and provide extra passing lanes and barriers along the route.

NZTA had previously investigated other options, which included the construction of new road alignments to the east and west of the existing highway, with two and four lane extensions.