SHADOW treasurer Joe Hockey was not telling the truth when he claimed days before the election that the Coalition's costings had been ''audited'' by a big accountancy firm.

His claim that ''we have the fifth-biggest accounting firm in Australia auditing our books and certifying in law that our numbers are accurate'' helped shield the Coalition from criticism about its decision not to submit costings to Treasury.

Documents seen by The Age show the federal directors of both the Liberal and National parties would have known the claim to be untrue.

Brian Loughnane and Brad Henderson signed a letter on the day the costings were released addressed to the Perth accountancy firm WHK Horwath confirming that its work for the Coalition was primarily ''not of an audit nature''.

Mr Loughnane also countersigned a letter from two of the firm's principals that reminded the Coalition their work would ''not constitute an audit in accordance with Australian auditing standards or a review in accordance with Australian auditing standards''. Within hours of the work's release Mr Hockey inflated it to a legally watertight endorsement, telling ABC TV ''they have certified our numbers based on all the information we have provided them; they have legal obligations and legal risks''.