Want to know what data Facebook holds on you? Stuff shows you how.

A Wellington software developer is urging others to download and check their data after a shock discovery of just how much Facebook collects.

Dylan McKay was stunned to find out the social media giant recorded his calls and texts history from outside the app.

All users can download their data from the setting option on Facebook and have the results emailed or downloaded directly onto a computer's hard drive.

DYLAN MCKAY/TWITTER Dylan McKay downloaded his Facebook data as a ZIP file and was astonished.

McKay said he became interested in what Facebook had on him after revelations that data mining firm Cambridge Analytica, working for the Trump campaign, improperly obtained data on 50 million Facebook users.

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When McKay tweeted his discovery, it was retweeted 36,000 times in five days.

Downloaded my facebook data as a ZIP file



Somehow it has my entire call history with my partner's mum pic.twitter.com/CIRUguf4vD — Dylan McKay (@dylanmckaynz) March 21, 2018

McKay said he was convinced the phone data was from his mobile service, not the Messenger app.

"Every single one of the call records is identified by a cell phone number, even prefixed by the +64 country code of New Zealand."

SUPPLIED Facebook displayed this message on phones, which enabled the company to store users call and text history.

​Netsafe chief executive Martin Cocker said he had not heard of others having their call data recorded, however he was not surprised it was.

"It's unnecessary, but what many data-gathering targeted advertising sales companies do is they capture any data that's technically within the sphere of the product they are delivering. We know that Facebook doesn't just record what you do on Facebook, it records what you do in and around Facebook," Cocker said.

"They do it because the more they learn about you the more accurately they can profile you, and therefore connect advertisers with you."

Cocker said he regularly advised users to download their own Facebook data, and it often surprised people to see how much was stored.

GETTY IMAGES Facebook's value dropped NZ$70 billion last week over the use of data.

Facebook was not subject to New Zealand privacy laws, which stipulated companies should not capture information they did not require, because they were based in Ireland and the United States.

"They're showing you what they capture, so they'll argue that's transparency.

"Every time you install new apps on your phone you have these very broad requests for permission to do quite wide-ranging things. Some app writers put those permissions in to give them room to operate, and others do it because they genuinely want to capture lots of other data. It's something people need to be more aware of than we currently are."

Alongside a record of photos and messages, McKay also found links to ads he had clicked on, and target words and interests for advertisers.

An undercover investigation by Channel 4 News reveals how Cambridge Analytica secretly campaigns in elections across the world. Bosses were filmed talking about using bribes, ex-spies, and fake IDs.

A lot of his stored data didn't come as a surprise, because in order for Facebook to show users their content he understood it had to be stored somewhere.

However, seeing his call metadata, including durations, and whether they were incoming or outgoing, was enough to make him uninstall the Facebook Messenger app.

"It doesn't seem like even for advertising it would be that useful... It's quite over-reaching."

Deciding what data Facebook should be allowed to retain was tricky, McKay said, but a simple test of whether the company actually needed it might be a good place to start.

Facebook shares slumped more than 9 per cent by mid-last week – knocking about NZ$70b off the value of the company.

A Facebook spokeswoman confirmed the data was stored, following permission being given after downloading the Messenger and Facebook Lite apps on Android.

"The first time you sign in on your phone to a messaging or social app, it's a widely used practice to begin by uploading your phone contacts. Contact uploading is optional. In Messenger and Facebook Lite people are expressly asked if they want to give permission to upload their contacts from their phone - it's explained right there in the app when they get started.

"People can delete previously uploaded information at any time."

Privacy commissioner John Edwards said last week that the organisation was monitoring the situation in the United Kingdom and had noted that his counterparts at the UK's Information Commissioner's Office, Office of the Privacy Commissioner of Canada and the US Federal Trade Commission have announced investigations.

"We will continue to monitor developments, but at this stage have not taken any decision to launch a separate investigation of our own."