I think you would just have to provide 3 written warnings that they are not performing at super star level.

They also have to go on a "performance management plan" where the employer has to make an effort to help them address any performance issues. There also needs to be very clear documentation of said issues, and how everyone is addressing them. The performance expectations also need to be documented AND reasonable in comparison to the industry, the company, the pay scale, and other similar roles elsewhere.

Unfair dismissal cases are surprisingly common in AU, although not often talked about. They can get VERY expensive especially for higher-paying roles.