National Australia Bank blamed the exit of a single large customer’s funds for a substantial chunk of the $4.2 billion in deposit balances leaving the bank in April as rival institutions compete for Australia’s deposit riches.

It is the largest month-to-month fall in deposits at NAB in at least five years, according to new Australian Prudential Regulation Authority data. NAB’s new chief executive, Andrew Irvine, has vowed to protect the bank’s long-standing reputation as Australia’s premier business bank.