HongKong - Haikou - Sydney

Juliana, your itinerary is definitely eligible for 24 hour visa free transit and you do not need any kind of visa. If you stay at Haikou airport in its transit zone, you do not need any documentation, but just your passport with a visa for Australia. However, if you want to exit the airport, your husband can do that easily because Australian citizens do not need a visa to enter China for up to 15 days. You are in different situation because you will need a temporary entry permit, so immediately upon your landing you will approach a transit counter, accompanied with your husband, where you will apply for a temporary entry permit to be allowed to exit the airport by presenting your flight tickets. It is up to the airport official whether you will be granted a permit or not, but since you are accompanied by Australian citizen, your chances are really very high. So, when you get a permit, you will both proceed toward immigration to spend some time in Haikou.