If someone posts something, or tells you they're feeling suicidal, and you think they're at immediate risk of harming themselves or attemping suicide, it's important to call emergency services (in Australia, Triple Zero: 000)

Immediate danger means that someone is at an immediate or impending risk of death or major physical injury. In this situation, it's important that the person gets help quickly. 

Even if they asked you not to tell anyone, getting them help is more important than keeping their secret. 

After you've called emergency services, try to contact a close friend or family member of the person to tell them what's happened. 

Make sure to look after yourself too. After you've informed their friend or family, do something to comfort yourself, and reach out to tell someone how you're feeling.