According to Australian Law, What is the rule about swapping shifts?

There is no law about something as basic as this. It is company policy which means you are scheduled to work at a particular time - if someone else turns up for that shift - the company is not obliged to pay them as they were not contracted for that shift. So any shift change has to be approved by the manager - if not then you will be listed as not turning up for a shift (and three strikes and you are out with most McD stores) and the other people will be sent home.