An anonymous letter exposing an alleged fraud at the National Australia Bank came as a shock to a senior bank investigator in the days before Christmas 2017, a Sydney court has heard.

Michael Griffiths, then manager of major crime investigations at NAB, told Sydney’s Downing Centre District Court on Tuesday the letter was “extremely unwelcome ... from my point of view” when it was forwarded to him on December 6, 2017, by a member of the bank’s whistleblower program.

“This sounds very selfish, but it was leading up to Christmas; I was extremely busy anyway,” Griffiths said. His initial reaction was one of shock, he said, but he gave the letter “my top priority attention”.

Griffiths was called to give evidence in the trial of Sydney businesswoman Helen Rosamond, who is standing trial on charges relating to her alleged role in defrauding NAB of $15 million between 2013 and 2017.

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Rosamond has pleaded not guilty to 60 counts of giving a corrupt benefit and 32 counts of dishonestly obtaining, or attempting to dishonestly obtain, a financial advantage by deception.

The court has heard Rosamond’s event management company Human Group organised leadership conferences among other events for the bank.

The Crown alleges some of Human Group’s invoices to NAB covered Rosamond’s personal expenses, including $228,747 for an interior designer to furnish her Potts Point terrace, as well as gifts for senior bank staffer Rosemary Rogers.

Rogers, a former chief of staff to the bank’s chief executives, has previously told the court she approved inflated invoices from Human Group as part of a fraudulent scheme with Rosamond.