If your store has a Facebook page, please consider the recent High Court ruling.
Now, if you post or share content on this Facebook page and allow comments, you may be responsible for the statements made even if you do not know of these comments.
One solution is that you can disable comments, but this is not what any of us want to do, so we are introducing on our Facebook page more surveillance in case.
Under the new defamation laws in some states, you must get a "concerns notice", and only then do they have 15 days before they can file a lawsuit. So you get some time to plead your case. You may want to check on this in your state. However, even if this is true in your state, I am unsure what happens if the affected person is in Western Australia and you are in NSW. In which case, you may not be given a grace period. I am not a lawyer, so I cannot answer these questions.
But, if you receive one of these notices, I firmly recommend that you take action.