Commonwealth Bank has admitted it lost historical bank statements belonging to almost 20 million personal accounts in a 2016 incident it chose not to make public.

It's understood two magnetic tapes that contained customer statements were scheduled for destruction by the bank when they disappeared in May 2016.

The tapes contained customer names, addresses, account numbers and transaction details from 19.8 million accounts spanning 2000 to early 2016. They did not contain passwords, PINs or other data which could be used to enable account fraud, CBA said in a statement on Wednesday night after BuzzFeed broke the story.

"An independent forensic investigation ordered by CBA in 2016 and conducted by KPMG determined the most likely scenario was the tapes had been disposed of. The bank immediately put in place monitoring mechanisms to further protect customers."