As motorway planners set the ball rolling today for a $430 million tunnel beneath Auckland's Victoria Park, they are being urged not to cut corners above ground.

The Transport Agency has already started advertising for expressions of interest from contractors interested in building Auckland's most expensive roading project - hard behind a Government announcement that a four-year construction schedule will be brought forward to next January.

It already holds all necessary planning consents for a three-lane northbound tunnel stretching for 470m below Victoria Park and the concrete motorway viaduct which will remain above it for the next 20 to 30 years, for the exclusive use of southbound traffic.

An extra lane each way will also be added to the surface motorway through St Mary's Bay, between the park and the harbour bridge.

The consents include provision for digging out 200,000 cubic metres of material for a "cut-and-cover" tunnel through reclaimed land where early Aucklanders used to catch fish from boats, before retiring to the Birdcage Tavern, a prominent foreshore watering hole since 1886.

They cover the hotel's transfer on skids to a site about 30m up Franklin Rd, as well as the removal of 86 trees and more than 100 church carparks near the tunnel's northern portal.

Four specimen London Plane trees are to be transplanted to new positions in the agency's reinstatement of Victoria Park, once the "lid" of the tunnel is in place for an almost doubling of traffic capacity by early 2014 through what is now a chronically constricted motorway corridor.

The existing northbound motorway on ramp from Fanshawe St will be reconfigured to pass over the top of the northern tunnel portal, and a footbridge will be built to the west, over a widened SH1 to the foreshore from the "Jacob's Ladder" public steps from St Mary's Bay.

Transport Agency northern director Wayne McDonald said last night that sports teams and other members of the public would still be able to use much of Victoria Park during the big dig, which would be confined to the western side, between Beaumont St and the existing viaduct.

But he refused to countenance a bid by a prominent urban design specialist, architect Richard Reid, for a lid over the tunnel's southern portal to be extended 10m to allow for the Birdcage to be given pride of place in a public plaza of the future.

Mr Reid said earlier he remained hopeful from discussions yesterday morning with an agency engineer that his proposal was being taken seriously, given the strong economic and social benefits he believed it would provide, once a second tunnel could be built and the 1960-era viaduct removed.

"It offers a richness at ground level in terms of the local community, retail and hospitality, in bringing people together," he told the Weekend Herald.

The agency's plan, to leave the Birdcage wedged behind the western side of the portal, was a recipe for "keeping people apart in an urban wasteland".

Mr McDonald would not comment on the architect's description of the official proposal, except to say it had passed muster at resource consent hearings before a joint panel of Auckland City and Auckland Regional Council representatives.