ANZ Bank has sacked a "small" group of its bankers for taking illegal drugs at a work-related event and recently let another staff member go for sexually harassing a woman from a rival banking organisation at a separate outing.

ANZ revealed the sackings in an all staff email sent on Monday by the bank's group executive talent and culture Kathryn van der Merwe that also detailed a massive surge in whistleblower cases at the bank.

Ms van der Merwe described the behaviour in her email to staff as "unacceptable".

"Earlier this year Shayne (Elliott, chief executive of ANZ) shared with line managers a true story where a small number of ANZ people used illegal drugs at an out-of-hours work-related event," Ms van der Merwe wrote.