I Googled number of eligible voters in Australia and got 15.7 million (2016), corrections welcome.
So hypothetically, if the 15.7 - 12.7 = 3 million who did not vote had voted against gay marriage, then the possible 4.9 + 3 million = 7.9 million (7.9 > 7.8) total who might have voted against gay marriage could have stopped it.
I suppose that we should also consider money influence from wealthy gay people as I understand happened in Ireland.
That's pretty close. The electoral roll on the day the papers were sent out was just over 16 million (16,006,180) That's partly increase in population over the year, partly because there was a concerted effort to get people to register for this vote.
So hypothetically, if the 15.7 - 12.7 = 3 million who did not vote had voted against gay marriage, then the possible 4.9 + 3 million = 7.9 million (7.9 > 7.8) total who might have voted against gay marriage could have stopped it.
Theoretically, yes. 48.84% of all eligible voters voted yes if you divide the Yes vote by total number of voters - so not quite a majority.
But it is almost certain that at least some of those who didn't vote would have also voted Yes - at least 94% of the non-voters would have had to vote No, to scrape a bare majority.
Elections are decided by those who bother to vote. But the Yes case almost certainly would have won even with 100% turnout.