Acts 2:21- "And everyone who calls on the name of the Lord will be saved."
As for the primitive tribes in Australia...
Matthew 12:14-18- " What do you think? If a man owns a hundred sheep, and one of them wanders away, will he not leave the ninety-nine on the hills and go to look for the one that wandered off? And if he finds it, I tell you the truth, he is happier about that one sheep than about the ninety-nine that did not wander off. In the same way your Father in heaven is not willing that any of these little ones should be lost."
And
Romans 1:19-20- " Since what may be known about God is plain to them, because God has made it plain to them. For since the creation of the world God's invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that men are without excuse."