Well, the American mustang springs originally from Spanish stock imported during the Conquest in the 16th century, and they're still classified as feral, so your kitties probably have a ways to go.
This is going to sound like nitpicking, but the two terms you're using are not equivalent. Feral refers to members of a domestic species which run wild; native refers to species which originated in the area in question, and the equivalent terms are imported or invasive. Given that our ideas about what is and is not native is based largely on human time spans rather than the geological time spans appropriate to species history, sorting out "native" from "imported/invasive" can be awkward. It's an important question, too, because - as your mom can tell you about those cats - an invasive species can be devastating to the native ecosystem.
Examples abound. All modern North American megafaunal species except the black bear and (probably) the bison are Asian imports - the grizzly, the elk/wapiti, the musk ox, humans. One of the numerous unanswered questions about what was going on in North America at the end of the Pleistocene is, did these megafauna cause, or merely exploit, the demise of the native species? (Bison are problematic, because the present shorthorn bison closely resembles the extinct Eurasian shorthorn bison, so it's possible that Eurasian bison replaced American bison antiquus instead of antiquus speciating into the present form.) All of these animals are presently considered native, because at the time Europeans arrived they had adapted to the continent and the continent had adapted to them.
Although the horse developed in North America, the present horses running wild in the western states are feral, because they are descended from domesticates, and invasive, because they are distinct from the horse species that went extinct here. The degree to which they fit into the natural environment is problematic, because their introduction coincides with the introduction of Western culture (despite the name, an import from the East!), which tends to alter the environment in destructive ways. The impact of the mustang on the environment is difficult to guage in an ecology which is constantly changing under a variety of pressures.
Other invasive feral domestic species, such as the hog, and imported invasive wild creatures, such as the starling and sparrow, do demonstrable severe damage to native species with which they are in direct competition and are anathema to people like me who are interested in maintaining a viable ecology. (All the same, by far the majority of the birds I support with my bird feeder are European house sparrows. Let he who is guiltless cast the first stone.) Another accidentally and naturally imported species, the cattle egret, exploits a niche not exploited by native species, and is treated as if it were native, even though we know it blew over from Africa in historic times.
Any biologists in the crowd feel free to correct me, but if domesticates can ever achieve wild status, it presumably would take a geologically long time and would be marked by speciation - i.e., sufficient biological changes that breeding with the cats or horses or whatever kept by humans is no longer practical. Since any dog can breed viable offspring with any wolf, the breeding test has its weaknesses, and I'm not sure at what level of reproductive viability speciation would be held to occur. Since it's not likely to happen without a prolonged period of isolation from the domestic cat, and since as long as new cats are added to the feral population from the domestic stock, the point may be an academic one as far as Australian cats are concerned.
The changes your mother notes may be sufficient, however, for the Australian feral cat to count as a new breed (as the changes in the American feral horse have resulted in a breed called the mustang). Domesticates produce breeds readily, in time spans comprehensible to humans, as a result both of breeding pressure and environmental factors; which is not surprising when you think about it.