NSW Police raided three properties in Sydney on Tuesday after being alerted to the alleged fraudulent payments by a NAB whistleblower late last year.

In a statement, NSW Police says: "Investigators have been told about a scheme whereby a person would allegedly receive corrupt commissions to authorise payments by a financial institution of invoices for a contractor, which were in excess of the agreed value of the contract."

It is understood the investigation revolve around payments to executive events firm Human Group, a long-term supplier to the bank, allegedly approved by Rosemary Rogers, former chief of staff to NAB CEO Andrew Thorburn.

Rogers resigned from the bank in December last year. At the time, an email written by Thorburn to the bank's board was leaked to local paper The Australian, which referred to Rogers' departure and a "lapse of judgment".

In a statement on the ongoing investigation, Thorburn says: "This matter has been investigated by us, and was reported quickly to Police, and we will continue to fully cooperate with them. The people under investigation no longer work with the bank, or are involved with us.

"There are no customers involved or impacted."

The police seized large amounts of documents and electronic storage devices during the raids for examination by the squad’s forensic accountants.

Financial Crimes Squad Commander, Detective Superintendent Linda Howlett, says: “Our Corporate Corruption Team has been working tirelessly to track payments and other funds to determine if an offence has been committed. From the information we have gathered so far, we know there are people who have significant information that may assist us, and we are appealing for them to come forward."