Tunnel excavation for Auckland's City Rail Link has finally started under Britomart Station - just as the scale of the $3.4 billion project looks set to be expanded.

City Rail Link Ltd chief executive Sean Sweeney said today's start on digging a 14-metre trench for the new rail lines under Britomart and Lower Queen St followed two years of "careful preparation work to protect Britomart Station's historic Chief Post Office building".

Contractors Downer-Soletanche Bachy will remove a five-metre layer of ground from Lower Queen St, then dig a single 25-metre-wide trench in Lower Queen St and two 10-metre-wide trenches under the station building by mid-2019.

The weight of the 106-year-old station building will be taken by a series of underpinning frames, protecting it from damage.

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After the tunnelling, the trench will be backfilled, Lower Queen St will be reinstated and the station will be rebuilt with new basement and ground floor levels.

City Rail Link chief executive Sean Sweeney oversees the start of digging in Lower Queen St with Downer Soletanche-Bachy project director Dale Burtenshaw. Photo / Supplied
City Rail Link chief executive Sean Sweeney oversees the start of digging in Lower Queen St with Downer Soletanche-Bachy project director Dale Burtenshaw. Photo / Supplied

Meanwhile, Auckland Council members held a closed workshop today to discuss a proposal by the rail link company to lengthen platforms in new underground stations and open a new access from Beresford Square to the planned station on Karangahape Rd, planned until now to be accessed only from Mercury Lane.

The extra work, expected to add hundreds of millions of dollars to the $3.4 billion cost of the scheme, would make room for nine-carriage trains, up from three- and six-carriage trains planned until now, boosting the line's capacity from 36,000 passengers an hour to 54,000.

Labour councillor Richard Hills blamed the previous National Government for not providing for longer trains and two entrances to the K Rd station, tweeting: "Why did the last council/govt cancel Beresford station."

But National Party transport spokesman Jami-Lee Ross said Auckland Council did not formally propose the expanded scheme to the Government before last year's election.

Ross said he did not have access to new projections showing that the new rail line could reach capacity if the scheme was not expanded by 2035, 10 years sooner than previously expected, but he was open to supporting the expansion upfront.

"We are in a wait and see mode on this. We are not going to oppose everything for the sake of opposing it," he said.

"If it can be proven that doing this work upfront will ultimately save the public and the ratepayer in the long term, then that's fair enough.

"I think it's important that they are transparent with the information and that they are open with taxpayers and ratepayers who are ultimately paying for this."

Labour's Transport Minister Phil Twyford said he agreed with the principle that the new rail line should be able to cope with passenger growth without the need to shut it for expansion work soon after it opened.

But he said he had yet to take a paper to Cabinet to approve the expansion.

The rail link company is half-owned each by the Government and Auckland Council and the line is due to be completed by 2024.