Science has been and always will be an international effort, if you shut out foreign researchers you might as well close down science in Australia. It doesn't matter whether a foreign-born researcher gets an ARC DECRA grant as that money goes to an Australian university anyway, funds Australian students etc., it's not like a DECRA grant goes to your own bank account and you can only spend it on yourself.
I've seen the opposite to you, researchers coming from overseas because no-one else has their skills inside Australia, training Australian students here, and then staying on as PR. Scientists who leave Australia still keep on collaborating with Australian researchers and universities, it's not like they burn all bridges. I co-supervise with quite a few overseas-based researchers who left Australia for career opportunities, they're still adjunct here.