The motorway forms the northern part of the 48-kilometre Western Ring Route. Once completed, the Western Ring Route will provide a strategic alternative to SH1 between Manukau City in the south and Albany in the north via SH20, SH16 and SH18. For more information on the Western Ring Route, visit www.nzta.govt.nz/wrr

The motorway will be constructed in three sections - Upper Harbour Bridge duplication, Greenhithe Deviation and Hobsonville Deviation. The duplication of Upper Harbour Bridge was completed in 2006 and the Greenhithe Deviation opened in December 2007. Construction of the Hobsonville Deviation started in September 2008 and will continue through to 2012.

The Hobsonville project consists of a 6 kilometre, 4-lane SH18 motorway from the end of the Northwestern Motorway at Hobsonville Road to the western end of Upper Harbour Bridge in Hobsonville; and a 3 kilometre, 2-lane extension of the SH16 Northwestern Motorway from Hobsonville Road to Brigham Creek Road in Whenuapai.

When completed, the motorway will provide an alternative route to Hobsonville Road to help reduce congestion on local roads and improve safety in the area. New interchanges will be built at Hobsonville Road, Trig Road, Brigham Creek Road and Squadron Drive and a new pedestrian and cyclist bridge at Clarks Lane. There will also be provision for future bus shoulder lanes.