With easy social media and other digital access platforms it is easy to contact employees outside their hours. If you expect them to respond outside hours and the communication is about matters other than their roster, you could open yourself to a claim under Fair Work provisions.
The best way to avoid any trouble is to make it clear, regularly, that while you may email or post to a private business employee only Facebook group, you do not expect an out of hours response from them. Having documentation of your advice re this may be useful in the event of a dispute.