Christian expert Steve Fogg is responsible for Communications at the largest Baptist church in the Southern Hemisphere. Speaking from Melbourne, he told Premier's News Hour that people need to review their online security straight away.
He said: "I'd advise anyone that's connected with any of the properties that Yahoo has, such as Flickr and Yahoo itself, to immediately change the passwords and if you can change your email as well - make it a really secure and very strong password.
"Do it straight away."
He was speaking after Yahoo revealed the details of half a billion of its users were stolen two years ago.
Names, email addresses, dates of birth and security questions and answers were taken in what's believed to be the biggest cyber hack ever.
Steve Fogg urged people not to give out too much personal information such as where they live or what they do on social media profiles.
"There's no such thing as privacy anymore online," he said.
Steve Fogg said it is up to Christians to bring positivity to the online world: "Go make disciples online as well as offline - Facebook, Twitter, Instagram - that is the digital water cooler of today.
"We see so much bad stuff online we have the wonderful opportunity of redeeming the online space."
Listen to Premier's Ian Britton speak to Steve Fogg here:
