Regulation 11(3) of the Workplace Health Safety and Welfare Regulations 1992 state:

A suitable seat shall be provided for each person at work in the workplace whose work includes operations of a kind that the work (or a substantial part of it) can or must be done sitting.

The regulations are quite clear in saying that a seat shall be provided and not just should be or may be. For many jobs, like checkout work, for example, you may want to vary between sitting and standing over the length of your shift. A suitable seat must be fully adjustable and easily moved.