Key Points:

Two more pieces of Auckland's Western Motorway bypass are falling into place, with a contract sealed on its Hobsonville section and work starting on a second Manukau Harbour crossing.

Transit NZ announced yesterday that it had awarded a contract for $163 million to HEB Smithbridge to design and build a 6km length of four-lane motorway through Hobsonville and a two-lane road running for 2.2km, north of the Westgate shopping centre.

That compares with an earlier estimate of $201 million for the contract. Construction will start in September and run until 2012.

But Transit regional capital works manager Sumi Eratne said the agency was still setting aside about $220 million to cover its overall potential costs, including land purchases and contingencies.

The motorway will be an extension of a 5.5km section which opened in December through Greenhithe to Albany as State Highway 18, the main link between Waitakere and North Shore Cities. It will be followed by the two-lane extension of State Highway 16 from the top end of the Northwestern Motorway to Brigham Creek Rd, offering a more direct route to Kumeu and Helensville and opening up land for the proposed Massey North employment centre.

North Shore Mayor Andrew Williams said the Hobsonville link would give a big boost to the whole Auckland region and he was looking forward to faster and more reliable trips to the airport once the full 48km western bypass was in place by 2015.

He believed that would take considerable pressure off the main harbour bridge, especially by reducing freight traffic, even to the extent that future travel demand may be satisfied by a tunnel exclusively for trains.

Transit's announcement on Hobsonville came as the first traffic diversions were planned for work on a new duplicate four-lane bridge across the Manukau Harbour and a widened motorway between Queenstown Rd in Onehunga and Walmsley in Mangere for $265 million - another big chunk of the western bypass.

The southbound motorway off-ramp to Mahunga Drive will close again from 9pm to 5am tonight and tomorrow night, when eastbound traffic will also be unable to travel over the motorway along Rimu Rd.

Some trees have already been removed around the Rimu Rd on-ramp, which will be widened for a bus priority lane over the existing bridge.

The bridge will eventually be for northbound traffic only. Its new twin will be built to the east, supported by eight sets of piers, three of which will be made strong enough to support a potential airport rail link running under both bridges.

Transit is still considering a push by the Onehunga Enhancement Society for reclamation of the Onehunga Bay foreshore for about $33 million, to create four new beaches while allowing the motorway to be moved west to make room for a "utilities" corridor on the other side.

The society says Transit and other agencies such as national electricity grid operator Transpower have an obligation to remedy damage to the foreshore from past developments.

But although a Transit study has confirmed the feasibility of the proposal, it says it will be hard to gain resource consents and has given the society until May to obtain support.