The New Zealand Transport Agency (NZTA) has announced the official commencement of work on the NZ$220m ($178m) upgrade of the Northwestern Motorway (SH16) causeway in west Auckland.

The new 4.8km Causeway Upgrade project involves raising and widening of the existing six-lane causeway and adding extra lanes between Great North Road (Waterview) and Te Atatu interchanges.

The project will prevent flooding on to the motorway at times of extreme high tides and weather conditions.

"By raising the causeway 1.5m to prevent flooding onto the lanes we’ll create a safer and more reliable route for all road users."

Scheduled to be completed in late 2016, the project is being constructed by the Causeway Alliance which comprises the NZTA, AECOM, Coffey, Fulton Hogan, Leighton Contractors and Sinclair Knight Merz.

NZTA Auckland and Northland state highways manager Tommy Parker said that the upgrade will reduce travel times for commuters, freight and bus users.

"We’re also aiming to keep the motorway open and flowing during peak times as we construct the project over the next three-and-a-half years," Parker said.

"By raising the causeway 1.5m to prevent flooding onto the lanes we’ll create a safer and more reliable route for all road users," he added.

Other key features of the latest upgrade project are widening the motorway between the Whau River Bridge near Te Atatu and the Great North Road Interchange to four lanes citybound and four/five lanes westbound.

Bus shoulder lanes will be extended and facilities will be improved for walkers and cyclists on the Northwestern shared path alongside the motorway.

The project team will be implementing special measures to lower the impact on the Motu Manawa Pollen Island marine reserve beside the causeway.

The new project is one of six connected infrastructure schemes to link the Northwestern and Southwestern (SH20) motorways as part of the Western Ring Route.

In addition to the latest Causeway Project Upgrade (SH16), the projects that will link the two motorways include Maioro Street interchange upgrade (SH20); Waterview Connection (SH20/16); Lincoln Rd interchange upgrade (SH16); Te Atatu Rd interchange upgrade; and St Lukes interchange upgrade (SH16).

These projects are aimed at improving both city and North Island regional transport connections.