Picture this: you're about to pay at the grocery store but a stranger comes up and swipes their card instead.

Or you're walking down the street and somebody you've never met asks you to hold a speaker for them while they go to the bathroom — next minute, music is blaring from the stereo and the owner is nowhere to be seen.

The situations may seem strange, but these acts are increasingly being orchestrated with a camera nearby, capturing reactions to be shared on social media.

So, even if the stranger didn't ask if they could use this footage, can it really be shared online?

Does a person need my permission to film me?

If you're in a public place, no.

According to Barbara McDonald, a professor at the University of Sydney Law School, there's no law against filming another person in public.

That's because there isn't a law in Australia that protects your privacy as an individual.

"If I’m just an ordinary individual then it’s difficult to say that privacy is protected," Professor McDonald says.

What if I don't want it on social media?

This is what happened to a Melbourne woman this week when she found out a video posted without her consent of a man leaving her flowers collected 57 million views and 10.9 million likes.

If a video taken in a public place without your consent is posted online, there isn't much you can do in a legal sense unless the video is defamatory or it's considered commercial.

But there are a couple of steps that you could take that aren't guaranteed to produce results, like asking the person who posted it to take the footage down, or you can report it to the social media platform.

What if someone's making money from this footage?

Professor McDonald says under our "passing off" law, the person featured in the footage needs to have a commercial reputation to be protected.

"If it's just some ordinary person, that's more difficult," she says.

Passing off protects business owners who don't have registered trademarks from wrongful misrepresentation of their products or services.

Here's an example from Professor McDonald to explain a commercial reputation:

"If you had a coffee shop and you took a picture of Ash Barty outside, it might be very obvious that you've just put this up and that she's just there and smiling and very happy to be photographed at your cafe," Professor McDonald says.

"On the other hand, if you started to say 'Ash Barty thinks our coffee is wonderful', you're beginning to make it sound as though she's endorsed your product, which is a different thing and it gets more complicated, that's someone with a commercial reputation."

I took a video of somebody I don't know and want to share it online, can I?

If you want to post the footage, you need to make sure it doesn't defame anyone and that you're not using confidential material, Professor McDonald says.

"Generally, defamation is the biggest risk for putting up a film of somebody — if it demeans them or portrays them in a very bad light that makes others think less of them, then that could be very problematic," Professor McDonald says.

"It's very easy to defame people."

Other than that, you legally can, so it's up to your discretion.

What's considered a public place?

Professor McDonald says a public place is anywhere you can move freely.

These are all considered public places:

• football stadiums
• grocery stores
• restaurants
• shopping centres
• concerts

So if you're in a crowded stadium watching the football, there's a high chance you might find yourself in the background of a photo.

There are, however, some places that might have entry conditions.

"if you go to a private restaurant, the condition may be that people not use their mobile phones because the restaurant wants it to be a place where people can come freely and don't feel like they’re going to be posted on other people's social media," Dr McDonald says.