Auckland Mayor Len Brown believes trains may be able to fit into tunnels across Waitemata Harbour, beneath motorway traffic.

He confirmed yesterday that he was keenly studying tunnelling developments in China where this was already happening, in his push for a rail link to Albany.

A report for the Transport Agency, issued on the eve of a mayoral planning summit held by Mr Brown yesterday, excluded rail from a comparison of costs between a possible new bridge across Waitemata Harbour, and a pair of three-lane motorway tunnels.

It estimated that the road tunnels would cost up to $5.3 billion, compared with $3.9 billion for a second bridge close to the existing "coat-hanger" structure which has served Auckland since 1959.

The report said a separate rail tunnel or tunnels would cost about $1.6 billion, and would be difficult to justify for about 30 years, during which it expected the Northern Busway to cope with rapid transit demand.

Engineering consultants considered incorporating rail beneath a three-lane road in each of two double-decked bore tunnels, but rejected the idea as they did not believe boring technology would provide for large enough tubes beneath the harbour.

Mr Brown said during a break in yesterday's conference that transport technology was advancing fast, and the Chinese had managed to accommodate trains and motorway traffic in shared tunnels.

"So technology will certainly be our ally and our friend," he said.

But a harbour crossing would be just part of a rail link to Albany, the cost of which an Auckland Plan discussion document issued for the conference said was likely to be far greater than Mr Brown's two other priority rail projects - a central city tunnel and tracks to the airport.

The document, which has invited Aucklanders to indicate their preferences for a new harbour crossing which the Transport Agency says will be needed within 20 years, did not put a cost estimate on an Albany link.

But it said indicative costs for the other two rail projects were about $2 billion each.

It said the choice between a new harbour bridge or road tunnel would be influenced by impacts on surrounding land use.