I'm not familiar with all your potential predators in your part of Australia but here if a chicken totally disappears without a trace it is generally a snake, canine, or human. A relatively large cat will often carry one off a bit and bury what is left for a later meal but usually leaves some feathers behind. Some canines will sometimes leave feathers. You may have birds of prey large enough to carry off a chicken but again they often leave feathers. Just because certain canines, cats, and birds of prey often leave feathers behind does not mean they always do but it can be pretty good clue.
If you can determine what it is you can perhaps trap or shoot it. That is not a final solution but it will stop critters that are actively hunting your area. Part of the problem with that is that if you have one critter hunting your area you probably have many. Unless you catch it in the act how do you know you actually got the one that is doing the hunting? I once shot 16 rabbits out of my garden before I finally got the last one that was eating beans as they sprouted. Not all those rabbits were eating the beans as they sprouted but it took a while to get the right ones. I've trapped critters on consecutive nights which means more than one was visiting. I once had a skunk go into my garage through a pet door and spray. Over the next two weeks I trapped seven skunks but don't know if I got the right one. The other issue is that new predators are being born, weaned, and set out to establish new territory all the time. Hunting or trapping will remove immediate threats but it doesn't help for future problems.
A more permanent solution is barriers. What barriers will depend on what critter. My general philosophy in this is to provide a predator-resistant area during the day and lock them in a pretty predator-proof coop at night when the danger is greatest. My biggest threat has been ground based predators, some of which can climb. I use electric netting during the day which has been successful for several years. Over a 10-year period I lost one to a hawk and another to an owl. Before I put up that electric netting I lost a couple to a fox over a three year period, then several at a time in two separate dog attacks. Sine I put up the electric netting all ground based attacks stopped.
That's my arsenal, trapping, shooting, and barriers. I don't have a dog out there 24/7. A dog big enough to handle your predators, trained to not harm the chickens, and that is kept out there 100% of the time is another layer of protection. If you bring that dog in the house it can't provide protection. It can only be in one place at a time so if it sleeps on your porch it might miss something behind the barn. But a good dog can be a powerful layer of protection.
One other thing I did before I got the electric netting. When I lost one to a fox I left the chickens confined to a fox-proof run for about a month. That seemed to make the fox think there was not an easy free meal at that location so it stopped hunting that area. That limited my loss to that fox to one, but I have a fox-proof run ready.
Good luck. Predators are not always easy.
