| Country: | NZL |
| Type: | SH |
| Route: | 18 |
| Map Custom: | yes |
| Length Km: | 14 |
| Direction A: | North-east |
| Terminus A: | at North Shore |
| Destinations: | Greenhithe, Hobsonville |
| Direction B: | South-west |
| Terminus B: | at West Harbour |
| Previous Type: | SH |
| Previous Route: | 17 |
| Next Type: | SH |
| Next Route: | 20 |
State Highway 18 (SH 18), also known as the Upper Harbour Motorway, is a state highway that runs between the Auckland Northern Motorway (SH 1) on the North Shore and the Northwestern Motorway (SH 16) in West Auckland, New Zealand. SH 18 forms the northern part of the Western Ring Route, which continues from Waitakere south-east to Manukau.
SH 18 begins at the Auckland Northern Motorway and runs southwest as a dual carriageway road, and is coextensive with the Upper Harbour Highway, which after two kilometres becomes a motorway at the Albany Highway interchange. This four-lane section of motorway opened in December 2007, and bypassed the single carriageway Upper Harbour Drive. The motorway heads south-west, crossing the upper reaches of the Waitematā Harbour over the Upper Harbour Bridge. At Squadron Drive, the highway extends onto a new section of motorway, which opened in August 2011, bypassing Hobsonville Road.[1] This road cuts across in a southwestern direction across farmland to Westgate, where it turns southward and merges with the Northwestern Motorway southbound.
SH 18 formerly ran along the Coatesville Riverhead Highway, further north than its current position. This route is now designated as Auckland Urban Route 28.
The route of the new motorway from Upper Harbour Bridge to Westgate was originally to receive a cycleway as well, but although the land was set aside, there is no timetable for the construction of the route.[1]