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Plans for an underground railway station bigger than Britomart are part of an inner-Auckland tunnel project being investigated by the Government and regional authorities.

Auckland City today celebrates the fifth anniversary of its showcase $211 million Britomart transport centre, for which it paid $146 million.

But regional planners believe a station deep below Albert St will put more passengers within easy walking range of shops, offices and classes.

"The central station will be larger than Britomart," said Auckland Regional Transport Authority planning and strategy general manager Peter Clark.

Although Britomart would retain interchange functions, connecting ferry and bus passengers with trains, a double-tracked station 18m below Albert St - beneath most of the block between Victoria St and Wellesley St - would be surrounded by "360-degree development".

That meant it should be designed to take crowds of up to 7700 passengers at its busiest morning hour.

The transport authority, which is helping the Government rail agency Ontrack to decide on the route for a 3.5km tunnel between Britomart and the western line at Mt Eden, believes circulating electric trains through an inner-city loop would more than double the accessibility of central Auckland to rail passengers by 2041.

It says the loop, estimated to cost $1 billion to $1.5 billion, would make the downtown area accessible by rail within 30 minutes to 370,000 people.

But Mr Clark said the project would have benefits well beyond inner Auckland, especially if rail links beneath Waitemata Harbour and to the airport were built, by encouraging residential growth around railway stations in centres such as Henderson and Manukau.

The authority's strategic planning manager, Maree McNeilly, said a forecast that the Auckland rail network would reach its capacity of about 23 million passengers a year by 2020 if Britomart remained a terminal station was made when petrol prices were half today's levels.

"We may hit capacity by 2011-12, so would like to do this sooner rather than later."

An underground rail link for Auckland was proposed in 1923 by Railways Minister Gordon Coates, and has re-emerged periodically in such forms as former mayor Sir Dove-Myer Robinson's rapid transit scheme of the 1970s.

The Auckland City Council commissioned an 18-month investigation of an inner loop project including two new stations during the previous term of Mayor John Banks.

The idea has since gained enough support in anticipation of electric trains for Finance Minister Michael Cullen to ask Ontrack to work on designating a route.