This isn’t just an issue in Australia, and not just for autism. As far as I can see, most countries will refuse permanent residency or citizenship if you or a dependant member of your family have a disability they think is going to be inconvenient.
Beyond the situations like this one, where we (as a nation) are shooting ourselves in the foot by reflexively denying some who wants to come here and whose skills we need, because they or their child is autistic (or a wheelchair user, or has Down’s Syndrome, or is blind, or Deaf, or even has disabilities that you wouldn’t notice unless you knew what you were looking for), it’s saying something to the disabled members of this country: it’s telling us that if the government didn’t have to, it wouldn’t let us in either. It’s telling us that we’re welcome* in our own country (*terms and conditions apply). It tells us that we’d better get used to how we’re treated here, because nobody else will take us either.