Depends on what kind of store it is, how big it is, and where you are. There are limits to how many people can be in a given store at once, and a lot of shops/businesses that are 'non-essential' are closed, but things like KMart etc are still open to the public, and at least some restaurants are letting people pick up takeaway orders in-store.
AFAIK the restrictions are mostly on a State or regional basis, e.g. I live in North-west Tasmania these days and we've just come out of three weeks of Stage 4 lockdown, so more stores were closed or else not open to the general public. You could still go into supermarkets etc. though.