Auckland should add more busways as far cheaper mass-transit options than rail extensions, says a visiting Australian transport expert.

Professor David Hensher, director of the Institute of Transport and Logistics Studies at Sydney University, says busways can be built for a fraction of the cost of new railways such as links to Auckland Airport.

"For every kilometre of heavy rail you build in Auckland you could do at least 27km to 50km of bus rapid transit on dedicated roads," he said at the airport, before being driven to Hamilton for the launch of Waikato University's new Institute for Business Research.

"When you look at Auckland, which is fairly low density, I'm absolutely amazed that you'd even consider heavy rail."

His comments will prove controversial after the release last week of a business case for a $2 billion-plus rail tunnel through central Auckland, which predicts hefty "transformational" economic gains from bringing trains within 500 metres of most parts of the inner city.

They also come as the Transport Agency is calling tenders for the first stage of an investigation into preferred routes for rapid transit and state highway links to the airport, and as Auckland Mayor Len Brown prepares this morning to celebrate a record nine million passenger rail trips taken in the region over the past 12 months.

The study - to be undertaken in conjunction with the Auckland Council, Auckland Transport, KiwiRail and the airport company - will consider both bus and rail rapid transit possibilities to cope with big predicted increases in air passengers and airport-based workers.

A 2008 consultants' report estimated an airport rail loop comprising links from both the north and east would cost $1.45 billion, although Auckland Council transport chairman Mike Lee believes an extension of the Onehunga line would be far cheaper.

Professor Hensher acknowledged there might be some merit in building the proposed 3.5km rail tunnel as "a little piece" of infrastructure to create a central city loop.

But he said the cost of travelling on Sydney's airport rail line made it cheaper for two or more people travelling together to catch taxis.

Auckland Transport planners argue that the entire regional rail network will benefit from the tunnel, because it will turn Britomart from a dead-end into a through station able to cope with 30 trains an hour - or 60 with enhanced signalling - compared with a maximum of 21 now.

They also envisage extending the reach of the rail network by adding more bus feeder services to suburban train stations.

But Professor Hensher said these would still have to compete with cars for road space.

He said busways should be far more prolific, even if it meant removing general traffic lanes to make room for them.

"We've got to get serious now about a lot more dedicated roads for buses into the suburbs."