Key Points:

Transit NZ will have to shelve an elevated motorway interchange at Onehunga if it is to duplicate the Manukau Harbour crossing - Auckland's main gateway from the airport - in time for the 2011 Rugby World Cup.

Auckland City Council commissioners have approved widening the motorway through Onehunga Bay for the $330 million bridge duplication project, but oppose Transit's proposal to build the interchange 7m above Gloucester Park and its Hopua volcanic tuff ring.

In a decision released yesterday, they have preferred a proposal by the Auckland Volcanic Cones Society for most of the existing interchange to be upgraded at ground level, but for its northeastern end along Onehunga Harbour Rd to be re-routed and raised above the railway line to join lower Galway St.

As well as softening the visual impact on the volcanic crater, that more direct route would ease concerns of local residents and businesses by diverting northbound traffic to the east of Onehunga Mall, which would then be closed to vehicles south of busy Neilson St, and replaced with a pedestrian and cycle link to the old Mangere Bridge.

It would also remove a blind corner on a tight bend considered an accident black spot.

The commissioners also want Transit to provide a "green bridge" public accessway over about 200m of the widened motorway, west from the Onehunga Bay Reserve to Seacliffe Rd, to join a shared pedestrian-cycleway to the old bridge.

That idea from Onehunga resident Colin Tunicliffe is likely to mean sinking the motorway into a "cut-and-cover" tunnel, a requirement Transit says it needs to consider before commenting on the feasibility.

The Auckland City commissioners sat with others from Manukau City and Auckland Regional Council in 11 days of hearings over applications from Transit for resource consents and notices of requirement for land outside its existing motorway designation.

Transit wants to duplicate its existing motorway bridge to provide four traffic lanes and a bus lane in each direction, and to widen the four-lane motorway along the harbour's approaches from Queenstown Rd in Hillsborough and Walmsley Rd in Mangere.

That would provide three lanes and a bus shoulder in each direction.

Although the commissioners could make only recommendations about the land designation, rather than a binding ruling, Transit would risk drawn-out appeals to the Environment Court by ignoring these.

Transit chief executive Rick van Barneveld acknowledged this would mean missing its target of completing the project by 2011, a scenario he was keen to avoid.

Although his board had 30 working days to decide whether to accept the recommendations, he believed the "most sensible way forward" would be to concentrate on providing extra capacity to the mainline motorway as soon as possible, and to make more modest improvements to the interchange in due course.

He declared that although Transit was disappointed by the interchange recommendation, and concerned about its long-term capacity to cope with growing traffic, "the substantial completion of the motorway widening and new bridge construction remains our target for 2011".

Volcanic society spokesman Greg Smith, whose organisation spent 18 months working with Transit to improve its motorway design around the slopes of Mt Roskill, welcomed the latest decision as serving a double purpose of satisfying community concerns and giving a higher priority to the Hopua tuff ring.

Maungakiekie Community Board chairwoman Bridget Graham said Onehunga had for too long been Auckland's Cinderella, denied ready access to its foreshore by the existing motorway, and she welcomed what she called a brave decision by the commissioners to listen at last to local concerns.

But the Auckland Business Forum, which includes the airport company, is demanding a "110 per cent commitment" by the regional and city councils to work with Transit and others to resolve all issues raised by their decisions, to ensure the duplicated crossing is ready in time.

Transit's $330 million proposal

* To widen a 5km section of motorway along State Highway 20 between Walmsley Rd in Mangere and Queenstown Rd in Hillsborough, and to duplicate its existing bridge across the Manukau Harbour.

* The duplicate bridge will provide room for four general traffic lanes and a bus lane in each direction.

* Motorway approaches to the bridge will be widened from their existing two lanes in each direction to three traffic lanes and a bus shoulder.

* The project is a key link in the western ring route between Manukau and Albany, and Transit wants it completed in time for the 2011 Rugby World Cup and the tens of thousands of visitors who will head across the harbour from the airport.