Auckland councillors want the Auditor-General to investigate the -City Rail Link's billion-dollar costs with the Government and Auckland Transport admitting the original $2.5 billion estimate is almost certain to change.
A letter co-signed by councillors Cameron Brewer, Dick Quax and George Wood has been filed with Auditor-General Lyn Provost, requesting independent scrutiny of the country's most expensive infrastructure project and what they say are "a number of red flags" for Auckland ratepayers.
Concerned that work on the huge underground project has begun before the final cost and agreement on how the bill will be split between ratepayers and Government have been finalised, the council trio have also cited Prime Minister John Key's claim that the CRL will "almost certainly cost more than they thought".
The Prime Minster's office said Mr Key's comment last week was based on advice he has received but refused to reveal any further detail.
The councillors say in their letter they are concerned about transparency and just how big a bill ratepayers are going to be hit with.
"We remain concerned that no formal funding agreement for this massive project between Auckland Council and the Government is in place, yet the physical construction has begun. We believe that a number of red flags remain for Auckland ratepayers."
The Office of the Auditor-General confirmed receipt of the letter and said it will now "fully consider the concerns raised".
Auckland Transport estimated CRL build costs of $2.5 billion and completion for 2023, with Mayor Len Brown saying he wants the council to negotiate a 50-50 payment split with the Government.
An AT spokeswoman denied the project's budget had increased but admitted "it is almost certain to change because the design detail has yet to be completed".
Asked about Mr Key's prediction of rising costs, the spokeswoman said he had "a very good understanding of the project and how major infrastructure works".
"Ratepayers and taxpayers should always be concerned that they are getting value for money.
"They can also have confidence in the robust level of scrutiny, planning and design that has gone into the CRL and the strong national and international team that has been hand-picked to get it to happen."
Transport Minister Simon Bridges said he was waiting to see more accurate costings.