Mr Cicutto said pre-tax losses on the unauthorised trading in foreign exchange options was $185 million, slightly above the $180 million loss it estimated last week.
But he said a revaluation of the foreign currency options portfolio might mean a “low probability” of a loss as high as $600 million.
Mr Cicutto said foreign currency options would continue to be part of the markets business for NAB.
“In view of the unauthorised activity that has occurred in this portfolio, the National is undertaking a market revaluation of the remaining portfolio to ensure it conforms with our policies,” he said.
“Our initial view indicates that this revaluation will lead to additional losses.
“Based on our work to date, there is a very low probability that the total losses will be as high as market speculation of $600 million.”
The NAB revealed last week that four rogue traders, since suspended, had made unauthorised trades that resulted in heavy losses.
The matter is the subject of four separate investigations, including an internal inquiry headed by Mr Cicutto, and an independent examination by accounting firm PriceWaterhouseCoopers (PWC) with the results not expected before the end of February.
The Australian Federal Police and the Australian Prudential Regulation Authority (APRA) have also initiated inquiries.
NAB chairman Charles Allen said the board had been updated on the investigations.
“Following the outcomes of the investigations we will take whatever action is necessary to ensure that investors, customers and staff have full confidence in the National,” he said.