Depends on who you have been called up by, whether it be coles' department stores or the supermarket itself...
Ive been to two interviews of their group interviews recently, so I speak from experience. Generally, they will be a mix up of different types of people, mostly young adults. You will be ordered to sit in a table group with these people, probably around 5 ppl per table. You'll be introduced to at least 3 observers, who introduce themselves. Afterwards you will be required to introduce yourself and why you want to work at the particular coles myer store, whether it be to your group or to the whole room. Next there will be a short presentation of the store itself - their rules, regulations, staff code, what the place is like etc etc. Then you will be required to participate in group activities with an observer sitting or standing near your group (be aware of his/her presence, their facial reactions to get subtle hints on qns and activities). The group activities themselves are group related and can include building a puzzle together, solving problems related to management, and customer servicing and role playing exercises, where you play as a salesperson trying to sell a product to a person looking for a particular item that you have to specialise in e.g. tennis racquets - this activity also works around to you being a purchaser.
tips:
Be vocal in your group, say anything that comes to mind and elaborate on it. There's no doubt that there will be some people in your group who would just stay quiet and murmur pointless ideas. Try to be a leader, by involving those who are shy. Always smile and respect your teammates, and resonate with your teammates' ideas by elaborating on their ideas. If possible try to elect yourself to speak out your group's ideas, when you are made to present them to the whole room.