A RESERVE BANK currency firm was willing to supply prostitutes and pay bribes to foreign officials to win banknote supply contracts, according to a federal police witness at the centre of the nation's most serious corruption investigation.

The explosive revelation is one of many made by a key witness in the Australian Federal Police inquiry into the Reserve Bank's polymer currency company, Securency International.

The witness has told an investigation by the Herald's sister newspaper The Age and ABC TV's Four Corners - to be aired tonight - that a middleman hired by Securency to win contracts from foreign governments told him that he intended to bribe a central bank governor from an Asian country.

The witness, who was a Securency employee, has given federal police his diary in which he recorded the middleman telling him in 2007 that the "governor would be very happy if the commission [payment] was increased".