Daniel Andrews' contentious deal with the militant firefighters' union could end up costing taxpayers nearly $700 million, with leaked documents suggesting the Premier may have misled Parliament – and the public – about the true price of the agreement.

Victoria's financial watchdog will be called to investigate, after Fairfax Media obtained figures estimating that the cost of the industrial deal could be in the order of at least $663 million – dramatically higher than the $160 million figure previously cited by Mr Andrews, Treasurer Tim Pallas and Emergency Services Minister James Merlino.

Voluntary and paid CFA members with Premier Daniel Andrews and Emergency Services Minister James Merlino in June. Credit:Penny Stephens

The indicative price tag is contained in a document crunched by the CFA's current chief financial officer, Nigel McCormick, and although it makes clear that the figure is by no means "final" it is nonetheless consistent with the numbers that have been previously been put to the Fair Work Commission, Treasury officials, and the office of the former emergency services minister, Jane Garrett.

And while the government disputes the data, the massive discrepancy with its own figures could prove damaging for Mr Andrews, whose economic credibility had already been dented after repeatedly claiming that the East West Link contract would "not be worth the paper it's written on".