Auckland City officers are urging councillors to back a $1.15 billion motorway extension through Avondale to Waterview, but want it kept largely out of sight of Oakley Creek.

The cost may be the destruction of many more homes than proposed by Transit NZ, to allow a "cut and cover" motorway tunnel down the western side of Great North Rd, but council staff point to environmental and social benefits for those who manage to escape the bulldozers.

A report due for council determination on Thursday supports connecting State Highway 20 to the Northwestern Motorway at Waterview, rather than an even costlier option of running it up the Rosebank Peninsula.

Transit, which strongly favours a Waterview connection both to complete a western ring route for Auckland and to funnel central-city traffic to the airport without clogging Spaghetti Junction and local streets, says the Rosebank option would cost up to $1.55 billion.

But the council report proposes, in return for supporting Waterview, a long string of conditions aimed at softening environmental and community impacts of Auckland's largest infrastructure project in 50 years.

It suggests the council will extract far more mitigation spending from Transit if it backs the Waterview option, rather than fighting the agency in the Environment Court and increasing community uncertainty from resulting delays.

Proposed conditions include bridging Oakley Creek in northern Avondale, but then running the motorway through a "cut-and-cover" tunnel on the opposite side of Great North Rd.

That differs from Transit's proposal to run the motorway on an open shelf between the creek and Great North Rd, and would mean the demolition of scores of houses to the west.

But council acting transport planning group manager Allen Bufton said yesterday that clearing more homes for the motorway would depend on the payment of fair compensation or the provision of replacement housing.

Transit spokesman Tommy Parker said earlier that each of his organisation's proposals would affect about 300 homes, and the Rosebank option would mean the demolition of about 20 commercial or industrial properties.

It would also have a greater environmental impact than Waterview, as the motorway would cross Heron Park and the Motu Manawa Marine Reserve on the eastern side of the Rosebank peninsula.

Mr Bufton said a reduction in traffic along Great North Rd, especially if Transit agreed to build a motorway interchange near its intersection with Blockhouse Bay Rd, should offer considerable environmental and accessibility benefits for remaining Waterview residents.

They now had considerable difficulty turning into and out of local streets against traffic from Great North Rd, which now carried 40,000 vehicles a day, more than half of which should be siphoned away by a motorway.

Mr Bufton said the council was also aware of strong community support for protecting Oakley Creek.

"We are very conscious of the community's love for Oakley Creek and [are] certainly going to do our best to get Transit to minimise the impact on that open space," he said.

City Vision councillors are believed to be divided over the alternative routes, although Citizens and Ratepayers Now representatives are likely to support Waterview, with strong backing from the Auckland Business Forum.

Mayor Dick Hubbard is also understood to favour Waterview, and wants a decision before Christmas to increase Transit's chances of accelerating the project to complete the ring route as early as 2011, ready for the Rugby World Cup.

That would be three years earlier than now forecast for the final link in a 35km route from Manukau to Albany, on which Transit says it will have to charge tolls to bridge an $800 million funding gap for that and other Auckland roading projects.

But city transport chairman Richard Simpson appeared last night to favour the Rosebank option as a better strategic traffic solution, and said he believed it would have less overall social impact than Waterview. He expressed misgivings that the officers' report had bypassed his committee en route to the council table.