Three Auckland councillors are asking the Audit Office to investigate a potential blow-out in the budget of the City Rail Link.
They're concerned the price-tag could rise from the current $2.5 billion - to $3bn.
Contracts for the major work on the Link have not been let, but the three councillors, Cameron Brewer, Dick Quax and George Wood, have written to the Auditor-General, asking her to scrutinise the process.
Auckland Transport chief executive David Warburton said the estimate had a 20 percent variance so the total cost could be higher or lower than $2.5bn.
He said there was a competitive tendering process and it was not in the public interest to talk further about the estimated cost.
Mayor Len Brown said there had been no new information on costs and expectations of the total price had not changed.