About 35% of Australians have subscription television. Not all of those viewers have BBC World News (I'd probably say most don't have it, although I'm not privy to figures), and news cumulatively - including Sky News, the most popular of those options - makes up only about 3% of all subscription television viewing, which in turn makes up about 20% of all television viewing.
1. 35% of Australians have BBC World already
This is a safe assumption. We shouldn't have to speculate wildly, anyway, because the ABC was kind enough to tell us:
I think ABC News 24 will re air ABC1 and their own primetime programming
"ABC News 24 timeshifts existing news and current affairs programs, including The 7.30 Report, Four Corners, Foreign Correspondent, Lateline, Landline and Stateline from all states and territories. Federal Parliamentâ€™s Question Time and repeats of Catalyst, Compass and other documentary and factual programs are also broadcast on the channel."