The Commonwealth Bank has issued a statement admitting it lost backup data on tape for more than 15 years of customer statements in 2016, affecting almost 20 million accounts.
It immediately sought to assure customers their information had not been compromised and no action was required.
The CBA's acting group executive for retail banking services, Angus Sullivan, issued a statement on YouTube after BuzzFeed Australia published an article about the incident on Wednesday.
"The tapes did not contain PINs, passwords or other data that could enable account fraud," he said.
In a statement the bank said it had confirmed there was no evidence of information being compromised for the 19.8 million accounts involved or suspicious activity following the incident.
CBA says it had been unable to confirm the destruction of two magnetic tapes containing historical customer statements.
The tapes contained customer names, addresses, account numbers and transaction details from 2000 to early 2016.
An investigation in 2016, when the incident occurred, determined it was most likely the tapes had been disposed of and the bank immediately put mechanisms in place to further protect customers.
"We take the protection of customer data very seriously and incidents like this are not acceptable," Mr Sullivan said.
"I want to assure our customers that we have taken the steps necessary to protect their information and we apologise for any concern this incident may cause."
He added that the relevant regulators were informed in 2016 but the bank had decided it was not necessary to alert customers after discussion with the Office of the Australian Information Commissioner (OAIC).