NAB target of alleged fraud by ex-employee

Police are investigating an alleged multi-million dollar corporate fraud at National Australia Bank that is understood to involve a former employee of the institution and a former supplier.

A police task force raided two businesses and a home in Sydney on Tuesday as part of an investigation into allegations that a person received corrupt commissions to authorise inflated invoices sent to the bank by a contractor.

Detectives believe tens of millions of dollars may have been taken over several years before the bank became aware in 2018 and notified police.

Financial Crimes Squad commander Detective Superintendent Linda Howlett said detectives were working closely with NAB's investigators to establish what has occurred.

"Our corporate corruption team has been working tirelessly to track payments and other funds to determine if an offence has been committed," Det Supt Howlett said.

NAB chief executive Andrew Thorburn issued a statement on the bank's website, saying police had executed a warrant involving a former supplier of the bank.

"This matter has been investigated by us, and was reported quickly to police, and we will continue to fully co-operate with them," Mr Thorburn wrote.

"The people under investigation no longer work with the bank, or are involved with us."

He said no customers have been affected.

It is understood the contractor was in the event management services business.

Police have not named the former employee but Det Supt Howlett said there are people with significant information that could help.

"These investigations are complex and often protracted but it's likely even a small piece of valuable information or insight could assist us in resolving the matter swiftly," Det Supt Howlett said.

Shares in NAB were unmoved by the news, gaining 1.3 per cent to $29.15 on Tuesday, in line with a stronger banking sector and broader Australian share market.