A problem with encryption software used by two-thirds of websites on the internet could be putting people's emails, passwords and banking information at risk.
Internet New Zealand is urging websites to update their software and re-issue security certificates to counter the threat.
The bug has been dubbed "Heartbleed" allows hackers to access private security keys used to encrypt and decrypt information sent to and from websites. They can then see private information users send to those websites.
Internet New Zealand said on Wednesday there is no trace left of the hackers' activities.
It says it is up to websites to fix the problem, and all end-users can do is avoid using the same passwords for multiple online accounts and change their passwords often.