When you use a virtual private network (VPN), it prevents your internet service provider from knowing what content or sites you are accessing. In theory.

But a recent analysis of 283 Android VPN-based apps on the Google Play store found more than 80 per cent of apps leak user data, while almost 20 per cent do not encrypt any user data.

The research, conducted by a team of researchers from the CSIRO, the University of NSW and the University of California, Berkeley, revealed that in some cases free VPN apps allowed malware or "malvertising" as "a deliberate way to monetise the app," by accessing personal information that could be sold to external partners.

After the team's findings were released, a number of apps were removed from the Google Play store.