Aucklanders should know by the end of this year whether a new Waitemata Harbour crossing may be provided within the next 20 years, says Transport Minister Steven Joyce.

Mr Joyce, who is also Associate Infrastructure Minister, said yesterday that he intended seeking advice from the Transport Agency on whether the Government should add the next harbour crossing to its proposed 20-year national infrastructure plan.

"We've got to make a call as to whether further development of a third harbour crossing [in addition to the existing bridges at Westhaven and Greenhithe] fits within that 20-year period," he told the Herald. "The Transport Agency and myself will need to form a view as to what is required here."

Consultants in a study last year for the former Transit NZ and Auckland councils recommended four bored tunnels - two for motorway traffic and two for passenger trains - as the main crossing of the future for up to $4.1 billion.

They recommended three-lane tunnels running 3.9km under the harbour for road traffic between Esmonde Rd and Spaghetti Junction, and single-track tubes for electric trains connecting either to Britomart or to a proposed underground inner-city rail loop beneath Albert St.

The existing main harbour bridge would be retained for local traffic only, with the addition of walking and cycling paths, leaving the tunnels to take over State Highway 1 duties.

Although the agency is already taking steps to protect the tunnel routes, and hopes to apply for subterranean rights before the end of this year, that is primarily to ensure that foundations for property developments such as around Wynyard Quarter are kept well clear of these.

It has been telling campaigners for walking and cycling links on the main harbour bridge that these are unlikely for at least 30 years, until tunnels are available for motorway traffic.