Mental Health First Aid
If you encounter someone who you believe needs a mental health assessment you can search for your locality and find mental health resources that are available for different age groups in your area. You can speak to the community teams/triage directly about your concerns or pass on the contact details as appropriate. Click on the link below.
If you witness dangerous behaviour, that is, a person physically harming themselves or others or placing themselves or others at risk of physical harm, then the local police need to be contacted. Depending on the situation you describe, the police may send members of their welfare team who have training in dealing with a variety of mental health and domestic issues. They will assist and make assessments on further action.