Journalist Nicky Hager is demanding answers from Westpac over revelations his private information was handed over to police during the investigation into his Dirty Politics book.
Over 10 months of his banking transactions were handed to police investigating the hacking of blogger Cameron Slater's computer without a production – or court order.
Mr Hager says he only knew about the "privacy breach by Westpac through court discovery" and he was unable to raise the matter with the bank, or the Privacy Commissioner until the matter had been raised in court.
He has asked for copies of the documents from the police under the Official Information Act and the Privacy Act – but that was refused.
Complaints were then laid with the Privacy Commissioner and the Office of the Ombudsman and representatives of those organisations have met with Mr Hager's lawyers.
He says he is "very concerned" by the Westpac breach and he "fully intends to explore all options open to him" now the details have been made public.
It was revealed on Saturday that police were given the transaction details by Westpac. Other companies, including Spark, Vodafone and Air New Zealand, refused to release information about Mr Hager's activity.
ONE News is seeking comment from Westpac.