Commonwealth Bank is defending its sacking of whistleblower and former chief medical officer of its insurance arm, Benjamin Koh, telling an inquiry he was dismissed for "serious and repeated" breaches of customer privacy that involved hundreds of documents being sent to his personal email account.

In a submission lodged with a Senate committee, CBA stands by its claim that Dr Koh, who went public in a joint Fairfax Media and Four Corners investigation into CommInsure, was not dismissed for raising concerns about misconduct in the business.

Dr Koh has filed an unfair dismissal claim against CBA, alleging he was sacked by the bank in August last year for being a whistleblower. He is expected to give evidence to the Senate committee at a hearing in Sydney on Thursday.

The CBA submission provides details of an investigation into what it says were serious breaches of customer privacy by Dr Koh, who had worked at the insurer since 2013.