Auckland Mayor Len Brown is being treated to a five-day fact-finding trip to China which he hopes will help his campaigns for major rail tunnels.

Although the Transport Agency is primarily considering building motorway tunnels under Waitemata Harbour, it is paying for Mr Brown's trip - which began last night - while saying it is also exploring rail options for the project.

It is spending $24,000 for the mayor and two of its own engineers, including northern highways manager Tommy Parker, to travel to China to inspect some of the world's largest tunnels and bridge projects.

The Auckland Council is also paying $8000 for travel and accommodation expenses of one of Mr Brown's staff, and Auckland Transport is understood to be spending $7000 for a senior infrastructure manager to join the party.


Mr Brown will also conduct civil duties, including meeting Mayor Wan Qingliang of Auckland sister city Guangzhou, as a precursor to a business mission he will lead to China in April.

A large bridge in Hong Kong is also on the itinerary.

The Transport Agency issued a report in March in which consultants estimated that a pair of motorway tunnels under Waitemata Harbour would cost $5.3 billion, compared with a new $3.9 billion bridge, and separate rail tunnels would cost $1.6 billion extra.

But it is understood that an 8.9km pair of tunnels and a 10km cable-stay bridge linking Shanghai with Chongming Island in the mouth of the Yangtze River cost little more than $2.5 billion to build.

The tunnels, which are among the world's largest with a diameter of 13.7 metres, have been built with three motorway lanes each, but with capacity for railway lines to be added below these at some future date.

Mr Brown said yesterday that he wanted to confirm the cost for himself during his trip, and was keen to discover whether Chinese technology could build tunnels in Auckland for less than the cost of a bridge, with capacity for rail as well.

"It may well be ... on a par or even cheaper than bridge building on the basis you are doing it that much faster, with that much more capacity," he said.