I found an interesting article on employment non-competition clause from Herbert Smith Freehills involving a fitness instructor in Hong Kong.
Such restrictive covenants are used to try to prevent employees competing during their employment and for a period of time after employment ceases.
What is reasonable in Hong Kong, will be different in Melbourne. What is reasonable in one industry, will be different in another.
Often a cascading clause is used. For example, a restriction against working in Victoria for 5 years, but if that is found to unreasonable then 20 km from Geelong and 2 years, alternatively 10 km and 1 year, and so on.
Every case is different.
You will find more information here.