Six Auckland councillors have joined mayoral candidate Wayne Brown calling for the books to be opened on a budget blowout on the $4.4 billion City Rail Link.

Brown wants City Rail Link Ltd to come clean about the finances of the 3.4km twin underground rail tunnel expected to transform the city's rail network before October's local body elections.

As far back as June last year, CRL bosses have signalled a rise in the price of the project, which Auditor-General John Ryan has called "the country's largest and costliest transport infrastructure projects ever".

Auckland Mayoral Candidate Wayne Brown. Photo / Greg Bowker

Today, Brown said he had asked for transparency on how much ratepayers can expect to be saddled with over and above the previous $4.42 billion cost "but they are afraid to tell us".

"In April they told us there would be an update later in the middle of the year on a revised cost blowout. Yesterday they said they were working towards the end of the year," he said.

Councillors Chris Fletcher, Tracy Mulholland, Wayne Walker, John Watson, Greg Sayers and Sharon Stewart and a number of council candidates, including former councillors Mike Lee, George Wood and former cabinet minister Maurice Williamson, believe the information is being held back for political reasons.

Watson said outgoing Mayor Phil Goff is leaving Auckland with "the mother of all hospital passes".

Said Fletcher: The scale of the rumoured cost increase is extremely troubling and could have a significant impact on the council finances, debt and rates. Full disclosure should be made to the ratepayers before the postal votes are sent out.'

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Councillor Chris Fletcher. Photo / Supplied

Lee said CRL Ltd's secrecy shows disrespect for the public's right to know.

"Not only are costs being poorly managed, so too is the project's end date which keeps disappearing into the distance," Lee said.

A spokesman for CRL Ltd said the body set up to deliver the project remains on target to make an announcement on costs and timetable at the end of the year.

He said CRL Ltd is not yet in a position to tell the council and the Government how much additional funding will be required.

CRL Ltd is working through a claim from the Link Alliance, which is responsible for delivering the main works, for Covid-related matters. Once the claim has been worked through and approved by the CRL Ltd board, it will be presented to the council and the Government.