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 Australia's most serious corruption investigation.

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

The witness has told an investigation by The Age and 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 the investigators 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."

Regarding the request to get a prostitute, the witness has revealed that one of the most senior Securency managers told him to arrange an Asian prostitute for a deputy governor of a foreign central bank.

"Next time that this official was in town, [I was told] that I was to procure him a bodyguard and with raised eyebrows and a wink... a particular type of bodyguard being an Asian woman," the witness told Four Corners.

"He was suggesting I might like to procure a prostitute for one of the central bank officials on his visit to Melbourne."

The witness says he did not act on the request although he believed other employees had arranged prostitutes for officials.

In a 2008 diary entry, the witness recorded that a consultant employed in Asia by Australia's overseas trade agency Austrade told him that to win contracts, Securency needed to hire someone to bribe officials or "to pass white envelopes for you."

Austrade this week confirmed that the Securency employee-turned police witness did report the comment to an Australian ambassador in the Asian country where it was made in 2008, but says that it had never been brought specifically to Austrade's attention.

Austrade also stressed it has never endorsed bribery by Australian businesses.

Secureny is a Melbourne-based polymer banknote company oversighted and half-owned by the Reserve Bank which aims to convince foreign central banks and governments to buy its special polymer banknotes.

An AFP taskforce is investigating Securency for allegedly bribing government officials in countries such as Nigeria, Malaysia and Vietnam.

Leading international figures - including former Malaysian deputy prime minister turned opposition leader Anwar Ibrahim and Nigerian central bank governor Lamido Sanusi - have urged the Australian Government to reveal how far the RBA's bribery scandal reaches.

"How could Securency allow ... huge bribes in the name of commissions?" Mr Ibrahim told Four Corners.

"It's something very difficult for me to comprehend. How is it a system [in Australia], with such a strong institution and respect for good government... could allow this?"

In a 2008 diary excerpt from the police witness, a Securency employee is recorded as telling him that that the Reserve Bank firm paid exceptionally high commissions to middlemen to secure a contract in Nigeria because so many people were "feeding off it".

"A range of senior government ministry officials and central bank officials would have been getting a slice of that 20-, 25-per-cent commission," the police witness said.

The Age has previously revealed that Securency paid millions of dollars into offshore accounts linked to mysterious middleman who helped the company win deals in Africa and Asia.

Some of the same middleman have been previously implicated in serious corruption scandals or have criminal convictions.

Last March, the Securency board released a seven-month audit of the company's dealings with its global network of agents, who have received almost $50 million in commission payments since 2003.

The KPMG audit is scathing of Securency management's conduct in appointing, monitoring and paying its agents, several of whom operate in some of the world's most corrupt countries.

After the release of the audit, the Securency board, chaired by RBA assistant governor Bob Rankin, announced the departure of the company's two top executives, Myles Curtis and John Ellery.

KPMG found Securency management ignored specific legal advice to treat requests by agents to be paid via bank accounts outside their country of residence as ''high risk'' and to subject them to a ''higher level of due diligence''.

A Reserve Bank spokesman says the RBA has nothing to add to its previous statements on the issue at this stage.

Earlier this year, the central bank said it endorsed the Securency board's move to implement all recommendations made by a review of its arrangements with its network of agents.

Nick McKenzie is a reporter for The Age newspaper on assignment at Four Corners.