'Social media can leave plenty of traces even if they are not on your phone.

"Social media apps can contain cached or copied content that is on the device's hard drive, even though the original content is principally stored on the social media company's servers," Cope said.

"Thus, when a device is in airplane mode, some of that content may be viewable on the device, even if it's otherwise private. As such, people can delete those apps for the purpose of travel and reinstall them later to avoid border agents accessing cached private social media content."

A New Zealand telecommunications expert who asked not to be named told RNZ that "I suspect if you've been online telling everyone what you think about Donald, it's too late" even if you delete information from your phone.

The expert said that the information is out there and seen by companies such as Peter Thiel-founded mass surveillance technology company Palantir.

"If apps are deleted on a phone but an officer knows ahead of time about a traveller's social media, the lack of the apps on the phone might lead to more scrutiny and questioning," Cope said.

"I had prepared," Kitchen said. "You go through the passport control and you do make sure that your social media has been cleaned up, that your phone is missing messages that might have been critical of Donald Trump, for example. ...In my case, it was not sufficient exactly because they had already done this background search on me."'