The Health and Safety at Work Act 2015 requires organisations to provide and maintain a work environment that is without risks to safety and health, including mental health, so far, a reasonably practicable.
Employers have a responsibility to ensure staff are working in a mentally safe work environment, and that they are taking action to reduce mental health risks in the workplace.
The Mental Health Foundation of New Zealand has some great resources for understanding the legal framework when it comes to mental health and wellbeing at work.