There are some reasons that an employee shouldn’t be fired. If an employee is fired for one of these reasons it is an ‘unlawful termination’.
See What’s unlawful termination? for a list of reasons that an employee shouldn’t be fired for.
Most people in Australia are protected from unlawful termination by the Fair Work Act’s general protections provisions.
However employees who aren’t in the national workplace relations system (such as employees of non-constitutional corporations in Western Australia) are covered by the Fair Work Act’s unlawful termination provisions.