It’s courteous to signal before turning left if you can do so safely but is not a legal requirement.
Want to know why? In Australia, your right brake lever operates the front brake, which is more powerful than the rear brake. When signalling right, you can use the rear brake to slow before turning. Using the front brake only can cause you to stop suddenly rather than slow and, which could be dangerous if you had only one hand on the handlebars. Signal left if you can do so safely or leave both hands on the handlebars for better control.