Key Points:

Auckland's booming Northern Busway is being eyed up for a $700 million to $1.2 billion extension to Orewa.

The Transport Agency is considering extending a two-lane highway for buses from Constellation Drive to at least Silverdale, superseding a previous plan to rely on motorway shoulders for trips north of Albany.

Its new proposal would involve building five tunnels and seven bridges, including a flyover of the Northern Motorway, leaving North Shore City and Rodney District to construct bus stations along the way - at Rosedale, Redvale and Silverdale.

Most of the busway would run along the eastern side of the motorway to Silverdale, from where bus priority lanes to Orewa are likely, whether along Hibiscus Coast Highway or through a new industrial zone to the north.

It would include a short section west of the motorway, looping through a tunnel from Rosedale to the Albany bus station, and then bridging State Highway 1 south of Lonely Track Rd to reach the east again.

The agency is even considering making the proposed extension capable of carrying a passenger rail line, which would put the project at the top end of the cost range. Its predecessor Transit NZ initially proposed a $500 million project including a 4.1km "off-line" busway extension to Albany and then shoulder lanes along the Northern Motorway for the remaining 19km.

But agency acting manager Tommy Parker told Auckland Regional Council yesterday that the busway's success with commuters since it opened in February, and a "distinct possibility" of extending passenger rail across Waitemata Harbour, had prompted a rethink.

"Continuing the off-line busway would build on the success of the Northern Busway to date," he said. "It would provide a world-class facility for the Auckland region and greater access and mobility throughout North Shore City and Rodney District."

Mr Parker, who is seeking support for $60 million to $65 million of regionally-allocated Government funds to be earmarked for property purchases to protect the route, said the extension was likely to be built in stages to match property development north of the harbour.

"The full busway is probably 20 years away but we believe there are going to be some localised hot spots, including Albany, which we expect to be built out in a few years," he told the council.

Although the existing 8.74km busway between the Harbour Bridge and Constellation Drive is along the eastern side of the motorway, buses have to cross that road to reach the Albany station.

Mr Parker said consideration was given to moving the station to the east, but planners decided it best to loop the busway through the existing facility before crossing back over the motorway.

An eastern route was preferable because of geotechnical challenges west of the motorway past Lonely Track Rd, and the fact that the Crown already owned much of the corridor to the east.

He acknowledged a risk of having to offer the land back to former owners as a key reason for early moves to protect the route.

As with the existing busway, the agency would be responsible for the route and local councils for designating sites for stations along the way.

These would include a park-and-ride facility at Silverdale, and possibly another at Redvale, depending on projections of traffic demand along the proposed $183 million Penlink road from Whangaparaoa Peninsula.

Regional councillor Joel Cayford said the previous Government's decision to build that road free of tolls would undermine demand for the busway.

"If you build Penlink, the busway will never happen - we can't afford to build both," he said.

Council transport chair Christine Rose also wondered how many commuters using Penlink would be prepared to leave their cars at Redvale and catch buses.

Mr Parker said the incentive, as development increased in Albany and elsewhere, would be to park up and use buses to beat growing motorway queues.