Transport Minister Simon Bridges is not saying if the City Rail Link faces another cost blowout, saying the "figures have moved around" and the goal is to come within the $3.4 billion budget.

The costings for the CRL have been redacted from Ministry of Transport briefing papers on Auckland transport funding - but they hint at rising costs for one of the biggest public projects in New Zealand's history.

Work is underway on the pair of rail tunnels from Britomart to Mt Eden, but the final cost will not be known until next year when the main contract to build the tunnels and stations is awarded to one of eight local and international firms competing for the work.

Asked why the costing figures had been redacted from the briefing papers, Bridges told the Herald the figures do move around as the Government and Auckland Council work through the details "but we are still working towards a $3.4b cost for this project".

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Pressed on whether he had received any advice from officials on the figures moving above $3.4b, Bridges said: "I think I would say they have moved around."

The briefing papers, obtained by the Auckland Ratepayers' Alliance under the Official Information Act, say "we cannot be confident at this point that the changes to the CRL cost profile will not lead to a reduction in other current planned transport expenditure".

The papers also refer to "project cost escalation" contributing to Auckland's rising infrastructure bill.

Auckland building costs rose 17 per cent in the past year compared with 7 per cent nationally.

Only 12 months ago, the city's top transport bureaucrat was assuring Aucklanders the rail link could be built for as little as $2b.

Auckland Transport chief executive David Warburton said in July last year the cost estimate for the 3.4km underground rail tunnel remained at $2.5b, plus or minus 20 per cent.

Two months later, Bridges and Finance Minister Steven Joyce revealed the cost had increased to between $2.8b and $3.4b.

Ministry of Transport papers showing CRL comments and costings redacted.
Ministry of Transport papers showing CRL comments and costings redacted.

Auckland Mayor Phil Goff said through a spokesman he was aware of the $3.4b cost for the CRL and had not received a briefing on cost increases.

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Last week, the Government and Auckland Council finalised the board appointments for City Rail Link Ltd - a joint Government-council company set up to deliver the project. The company is chaired by well known 'Mr Fixit' and former NZ Post chief executive Sir Brian Roche. The other directors are Russell Black, Brian Harrison, Karen Jordan and Anne Unwin.

Last year, the Government agreed to pay half the cost of the CRL. The council is paying the other half.