I don't think Australia was in the mind of the writer of Acts 1:8
I agree with you about that. Luke (the normally attributed writer of Acts) probably did not know of the existence of Australia (or many other parts of the earth for that matter). On the other hand, Jesus, to whom the saying in Ac 1:8 is attributed, would have had some understanding of the size of the planet and how it's land masses were distributed (assuming he was who the NT claims him to be - Jn 1:3, 10). So when Jesus speaks of the gospel being preached to the "most distant part of the earth" there would be good reason for thinking he meant it literally.
And we have the history of Christianity that backs up the fact that the gospel has reached every part of the earth. Not that all proclaimed Christians have backed up the good news with their fine conduct. Often, just the reverse. But even that was expected beforehand. (Mt 13:24, 25, 30, 47, 48)
Paul (at Ac 17:30, 31) also explained that Jesus' death and resurrection had changed God's relationship with humankind. According to Paul, following Jesus' resurrection, God was calling for "all men, everywhere" to "repent" because a day of judgment was coming. If one understands that day of judgment to still be ahead of us, then, in all fairness, the good news would need to be proclaimed "to the most distant part of the earth" in order for "all men, everywhere" to hear it and choose how they would respond.
I am not trying to argue with you Joen. Nor am I trying to support what the WT believes, although, I don't disagree with everything the WT holds to. For some years now I have tried to fashion my beliefs apart from WT influence. But I also don't automatically disagree with the WT just to be disagreeable with them. I am just stating how I understand things. And I also understand that, like the car commercials used to say, 'your mileage may vary.' So I respect your right to view things differently.