Again, this is standard language that you'll see in many statutes. It is to prevent them from being sued “for anything done or said in good faith in the exercise or purported exercise of a power or in the performance or purported performance of a duty or function of the Tribunal”.
The only way they can be sued is if they're acting outside the scope of their duties, and that also brings in the notion of doing something that is really not appropriate. They can't be sued as long they're in “good behaviour”.