Half a world away in Canada, watching on a live stream through the Musical.ly app, was a woman we'll call Tina.

Tina watched in disbelief and then horror as the girl began to cut herself, and threatened suicide.

This happened just two weeks ago.

Desperate to do something, Tina contacted the app's support email. She says she received a reply thanking her for drawing it to their attention and saying they would "message the user".

The next day, her iPad buzzed again, notifying her the girl was about to start another live stream.

To Tina's dismay and fury, she saw the child cut herself again.

When she contacted them again, the app's support team told her they would delete the account.

"What? That's it?" Tina says. "So at this point, I took it upon myself to do something."

Tina is a G-rated social media celebrity with hundreds of thousands of followers, many of them young kids. She has had many approach her online for help when they are bullied or abused in the apps they use. She has asked us not to use her real name.

She had been troubled by some particularly nasty strains of bullying showing up in her feed on Musical.ly involving what looked like very young kids, and began following some of the accounts.

One was the Australian girl's, which is why she was notified when the girl began to use the livestream.

"The internet is a very scary place, but it can also save someone's life," Tina says.

From the child's email, she was able to figure out what school she went to - and found out it was in Brisbane.

The app's administrators and the Canadian police wouldn't help, so she tried something else.

She found the name of Susan McLean, an Australian cyber safety specialist, and emailed her urgently asking for help, with all the information she could find about the child and screenshots of the self-harm.

Ms McLean tracked down the girl's school and informed the principal.

The principal responded immediately, seeking more information, and assured Tina and Ms McLean they would do everything possible to help her. The school had not known she was in such distress.

The internet's backwaters are full of terrible stories, but Musical.ly is a free, mainstream iPhone and iPad app with 100 million users that's marketed at teenagers.

Cyber safety experts and schools are increasingly worried about the popularity of the app and its new live stream function, Live.ly, among tween and young teen users.

Wenona's junior school banned it from school devices this week, and Fairfax Media has learned that Kincoppal Rose Bay also did so last month.

"It's dangerous in the extreme," Ms McLean says of the app.

"Sex crimes detectives are getting jobs from it where kids have been groomed. Dumb, ill-informed and head-in-sand parents want to pander to their kids and let them have every app they want because they have no understanding of the reality."

She says that while kids under 13 shouldn't be on any social media, Musical.ly has a litany of features and functions that make it a particularly unsafe space for younger kids, in addition to 18+ musical content.

The app publishes location information, videos can be downloaded from the app, and there is a lot of bullying in the comments as well as reports of hashtags "hijacked" with pornographic content.

"Friends commenting that you have 'ruined' their favourite song and [there's] bullying in real life as a result," she says.

"Predators being able to watch the videos of your child, take them and store them for future viewing pleasure."

Another cyber safety consultant, Leonie Smith, says the app's administrators are not doing enough to stop young users.

"It's one of the very first social media sites that I've seen that has taken off mostly in primary schools. High school students laugh because it is seen as very tween-oriented social media," says Ms Smith.

"The fact is anyone can message any of those kids on there, download the videos and share them really easily."

Tina runs popular accounts on several different social media platforms, but has been most disturbed by what she is seeing on Musical.ly.

"Kids on this platform are susceptible to bullying, harassment, having their personal information stolen or copied and also to solicitation for ads ... and accounts claiming to help them 'win fans'," she says.

She says anyone can watch the Live.ly livestreams, even without having to follow a user's account, meaning kids can broadcast from their bedrooms to anyone online.

A spokeswoman from Musical.ly, based in San Francisco, did not address Fairfax Media's questions about the self-harming incident.

"We take appropriate measures to expeditiously remove offensive or inappropriate content from the Musical.ly app.

"Users can set their account to private to ensure only the friends they know can follow them," she said, pointing out that users can block other users and report abuse.

"We are also implementing machine-learning technology to scan messages to block spam and inappropriate content."

Despite all the concerns, there is a very good reason Musical.ly is so popular with its users, known colloquially as "musers".

"This app is really fun," says Tina. "It can leave you in tears from laughing."

But, she stresses, "it's not for kids. There is not nearly enough protection within the app for kids. Making your account 'private' is not enough.

"[The app's makers] just need to think more about the safety of their users rather than the safety of the money in their pockets."

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• Kids Helpline: 1800 55 1800