The Consumer Council on Thursday warned that the personal data of people who use apps to block spam callers could be at risk because some of these apps pass on the information to third parties.

The watchdog investigated five such apps and found that two of them uploaded details from the user’s list of contacts onto their databases.

The council found that third parties could then access sensitive details including addresses and the monthly rents the contacts paid.

It's suspected these were extracted from the contact lists of landlords or property agents who also used the apps.

The council found that one of the apps would also access up to eight items of personal data, including names, photos and dates of birth, when registered via Facebook.

Speaking at a press briefing, Kenneth Chan, the Vice-Chairman of the council's Publicity and Community Relations Committee, reminded users to stay vigilant when using these apps.

"Consumers should pay attention to the following when using blocker apps. Read the terms of service and privacy policy before use, and evaluate whether the blocker app is trustworthy and the permission is reasonably commensurate with the functions offered," he added.

Despite the privacy risk, Chan also called on users to proactively report spam to help apps identify and block the latest telemarketing and spam calls.