@Kit26 In the UK the legal situation is different depending on whether you are able to host or not. If able to host but the guest can't travel that seems to mean that legally the guest has to pay. If the host is legally unable to host then the guest should be refunded. I think Aussie law is similar.
Of course many hosts do not think it right to charge if a guest doesn't stay but this is for individuals to decide provided they can legally host guests.