It is not legal in most countries to disrupt someone's success, but mental stress does cause disruption. Thus the question, is this illegal? and in which forms?
Depending on the degree of stress. A court has to observe the stress objectively as in based on the reasonable person and if it has caused damage psychologically. It does not have the resources to do otherwise and analyse subjectively by looking into their minds.
I do not believe it is ‘illegal’ however, I think it acts more as a civil wrong at this point in time, depending on the country you are in. Illegal would imply that it can be trailed criminally which in some cases might not be possible.
As for Australia, it would come into question under the fair work act when it comes to employment matters and the obligations on the employers to ensure the health and safety of employees.
I'm not aware of any laws explicitly making a criminal offence of causing someone stress.
Undoubtedly stress can be a consequence of civil wrong or criminal behaviour such as workplace bullying, sexual harassment, stalking, mugging, burglary, extortion, assault, fraud... the list goes on. But the defendant isn't accused of causing stress, they're accused of the bullying or burglary (or whatever).