Banking Ombudsman Deborah Battell is advising against online banking or emailing over unsecured public wi-fi networks after a six-figure sum was stolen.
An investigation by her office has found the money was removed when an email account was hacked and personal details were taken over an unsecured network.
The victims were an overseas couple and the stolen money was a term deposit in an unnamed New Zealand bank.
The theft occurred after the couple used public wi-fi at an American airport and a hacker accessed their email to forge a signature and get bank account numbers.
Ms Battell says while it was a rare case, the New Zealand bank's security practices were out of step with those of other banks.
The bank has reimbursed the stolen money and upgraded its security procedures.