Transit New Zealand has confirmed its readiness to dig a "partial" tunnel under Victoria Park in Auckland to clear one of the country's most notorious motorway chokepoints.

And the national roading agency is not ruling out eventually building a second tunnel under the park, once it completes studies on the route of a new harbour crossing costing up to $3 billion.

The agency was guarded until yesterday about its intentions, having yet to discuss them with local body leaders.

But it has since confirmed the Herald's disclosure of a decision by its board last week to support a one-way tunnel as a first-stage solution to chronic traffic congestion between the harbour bridge and southern motorway.

It is arranging meetings next week with the Auckland and North Shore city councils and the regional council to discuss the proposal, which could take no more than about five years to plan and build, depending on resource management approvals.

The tunnel, which it hopes to build for no more than $200 million to carry northbound traffic, will double the capacity of the existing four-lane viaduct over the park while leaving that structure in service for the next decade or so.

Transit had previously favoured an "iconic" replacement viaduct costing about $160 million rather than a six-lane tunnel running both ways for up to $290 million, unless regional funds could be found to fill the gap.

But the project has been stalled for two years by opposition from the Auckland City Council and Auckland Regional Council, concerned about the impact an enlarged flyover would have on Victoria Park.

The Transit board has now agreed to a compromise proposal, for which it is satisfied Government funds are available and which would leave the existing viaduct to carry southbound traffic in the medium term.

It has also left open the possibility of adding a second tunnel below the park, once it has a clearer idea of the route of the next harbour crossing and whether this will go under or over the water.

Detailed scoping work on the next harbour crossing will start in the New Year, but it will take at least 16 years for a new bridge or tunnel to be built across the harbour.

If it is to be a bridge, an indicative study has identified a route west of the existing harbour bridge, but a tunnel to the east may have to reach under Victoria Park as well.

The initial tunnel would run for about 600m, just west of the existing viaduct, north from Wellington St to the Assembly of God Church carpark next to the Fanshawe St on-ramp to the motorway.

A southbound tunnel would present more of an engineering challenge because of a relatively steep climb out of it.

Transit's acting general manager of transport planning, Wayne McDonald, insisted that a partial tunnel had always been within Transit's range of options, despite the two-year planning holdup. "We always talked about a full tunnel option or a partial tunnel option," he said.

Despite suffering from "concrete cancer", the existing viaduct could last up to 50 more years with remediation, he said.

But a one-way tunnel would "get us all going" for now, linking up with $195 million of improvements to the central motorway junction, of which the completion of the first $55 million stage will be marked by Transport Minister Pete Hodgson at a ceremony tomorrow.

Although Transit had not previously planned to start replacing the viaduct until 2009, it is under pressure to keep the momentum going on the junction improvements.

Mr McDonald said resource consents for projects of such a scale could be shortened by goodwill and acceptance at a local level.

St Mary's Bay Association chairman John Hill, whose organisation strongly opposed a replacement viaduct, yesterday welcomed the compromise, but still hoped the motorway could eventually be fully buried under Victoria Park.