The State Highway 20 extension project in Manukau is a motorway extension in New Zealand (NZ) designed to bypass the city between Manukau and the North Shore. It was launched by Transit New Zealand, in conjunction with the Manukau City Council.

The SH20 Manukau Extension was designed to help relieve congestion on key roads around the city centre such as Cavendish Drive and Great South Road where further retail, business, apartment and entertainment developments are expected. The new road replaced Wiri Station Road as the main link from Manukau city centre to the Southwestern Motorway.

The SH20 Manukau Extension provides the southern entrance to the 48km Western Ring Route. In turn, the Western Ring Route links the Southern Motorway at Manukau city centre via the Southwestern Motorway (SH20) (at the Puhinui interchange), the Northwestern Motorway (SH16) and the Upper Harbour Motorway (SH18) before joining the Northern Motorway (SH1) at Constellation Drive.

Concurrent with the motorway works, Manukau City Council had undertaken a separate project to connect Nesdale Avenue with Cavendish Drive.

The $210m extension project commenced in June 2006 and was completed by January 2011. According to the NZ Transport Agency, the new motorway played a crucial role in making the 2011 Rugby World Cup a successful event, which the country hosted.

Contractors

The design and construction of the project was carried out by Leighton Works, a joint venture between international construction company Leighton Contractors and New Zealand-based Works Infrastructure.

Leighton Works had contracted engineers Sinclair Knight Merz (SKM) to complete road design, Golder Associates for geotechnical design, EDAW Jasmax as urban design consultants and Hegley Acoustic Consultants for acoustic design.

“The SH20 Manukau Extension was designed to help relieve congestion on key roads around the city centre.”

Works Infrastructure supplied more than 150,000t of asphaltic concrete from their Auckland plant for the road surface.

Rail and air links for Manukau city centre

The SH20 Manukau Extension also provided some preparatory work towards the construction of a rail link to the Manukau city centre, adjacent to the motorway. The Manukau rail link is a 2km connection between the existing Main Trunk Line at Wiri and a station at Manukau city centre, improving passenger services in the city.

The project was developed jointly by KiwiRail, Auckland Regional Transport Authority (ARTA) and Manukau City Council. The whole project was finished in January 2011.

Before the commencement of the extension project, patronage of these services had risen by 70%. Manukau City Council also viewed the SH20 Manukau Extension as a critical connection to support future growth at the Wiri and Auckland International Airport business zones and an expected increase in population of 40,000 people at the new nearby suburb of Flatbush.

Nesdale interchange

“The SH20 Manukau Extension provides the southern entrance to the 48km Western Ring Route.”

Nesdale Avenue was extended to pass under the new motorway and across a diversion of the Puhinui Stream from where it joins the existing SH20.

The Nesdale Interchange is a major exit and an entry point for traffic to and from the new motorway.

Construction of the interchange was carried out in five stages to maintain traffic flow past the work site. This was one of the first projects undertaken in the construction of the new road.

Rail crossing

Twin bridges have been built to cross the North Island Main Trunk Rail Line. Each bridge has four spans totalling 120m in length. Earthworks formed a platform alongside the motorway for the future rail link to Manukau city centre. Work on the bridges started in mid-2008 and continued until July 2010.

Lambie Drive interchange

Traffic to and from Manukau city centre uses the interchange as a major access point for the completed motorway in both directions. It is also the main exit and entry point for Manukau Central Business District traffic to get to the Southern Motorway (SH1). A bridge was built to allow Lambie Drive to pass over the new motorway.

There is also a rail underpass for the future rail link to Manukau city centre. During construction, the traffic was handled in shifts in Lambie Drive to allow the work to proceed. Work at this site started in early 2008 and was concluded in early 2009.

Barrowcliffe Place and Great South Road

Work at Barrowcliffe Place allowed direct bridge access from the undeveloped area south of Wiri Station Road across the motorway to Manukau city centre. It includes a local road bridge and pedestrian access (work was started in 2007 by Manukau City Council).

A bridge was built at Great South Road to allow the new SH20 Motorway pass over it. The bridge was constructed in two stages. Traffic has shifted around the construction at various stages.

SH1 interchange

The SH1 and SH20 motorways join at the SH1 interchange. The interchange features a 240m-long curved flyover with ten spans. The exit ramp from SH1 to Great South Road has been realigned. Now the traffic travelling to Auckland from the south has direct access to SH20, to the airport and the Western Ring Route. Traffic from SH20 is now able to join SH1 travelling both north and south. This interchange was constructed in three stages. Construction work started in early 2007 and continued until late 2009.

Motorway statistics

There are three interchanges with motorway-to-motorway connections to SH20 and SH1 at Manukau City. The project involved 13 bridges and 268,000m² of pavement across 140ha. It required nearly 920,000m³ of earthworks. The work also included five motorway crossings over local roads and streams, as well as the main North Island rail line.

Western Ring Route

The new SH20 extension forms an important entrance to the southern section of the Western Ring Route. When the Western Ring Route is completed in 2019, it will allow traffic to completely bypass Auckland and the Harbour Bridge by linking the Southern Motorway (SH1) at Manukau City to the Southwestern Motorway (SH20), the Northwestern Motorway (SH16) and the Upper Harbour Motorway (SH18) before rejoining the Northern Motorway (SH1) at Constellation Drive.