Internet NZ expects New Zealanders will have been caught up in a huge Facebook hack.

The social media firm says hackers stole digital login codes allowing them to take over nearly 50 million accounts in its worst security breach ever.

A spokesperson for the advocacy body Internet NZ, Andrew Cushen, says the hack seemed to allow access to private parts of a user's Facebook profile.

He says it was another reminder that people needed to take internet security carefully.

"It's also and more importantly a call to action for organisations like Facebook who we trust to look after a huge amount of information about us all and it looks like this hack exploited part of Facebook's security set-up in a way that I hope they learn from quickly, I hope that they close that loop hole quickly."

Andrew Cushen says people should make sure they updated their passwords regularly, and make sure they were strong.

The company founder Mark Zuckerberg has described internet security as 'an arms race' and said Facebook was constantly working on it.