I don't want to be pedantic (read: I love being pedantic more than anything else) but I solely take issue with the word "always."
Because "we have always been here" can mean one of two things:
1. You believe that Aboriginal groups have literally always been there, since the formation of earth/the universe. This is patently absurd and so not a reasonable thing to suggest anyone believes.
2. You believe that from the moment homo sapiens sapiens existed, they existed in Australia. This is, also, patently absurd, but for some reason people seem to think it's possible.
Australian Aboriginals migrated there from Africa. Just like South American and North American indigenous groups did, to their respective locales. Many North American groups also claim to have always been there. It doesn't mean they were.
They migrated an enormously long time ago. Like, I'm willing to go as far as to say it could be within the first few hundred years of humans being humans.
But that isn't "always". And it means they didn't originate separately from the rest of humanity.
In Canada, we use the term "since time immemorial" which, I think conveys the point much better. The groups have been there so long that figuring it out beyond "a long ass time" is basically unimportant aside from archaeological curiosity. No one has a "better" claim to the land.
But saying "always" opens you up to pedantry, which can and will be exploited by bad actors.
It is, also, I think, a dangerous argument to make that Indigenous groups are different and special from other groups of humans on some fundamentally baseline level. Because, the racist would ask, if Aboriginal people really did originate differently, are they proper humans, or a savage subspecies?
Humans are humans, from the same origin point. Groups left at vastly different times, but we all left.
Edit: here's a really simple, hypothetical, explanation for why there are no oral histories of travel: because it happened so far back that it was before oral histories existed. That allows for us to accept oral histories as a reliable system of documentation, and explain it: by the time stories and traditions began being passed down, no one was around to pass that story down. I am sure there are many other explanations that allow oral histories to retain credibility while explaining this apparent gap.