The Chevy SS' shark fin antenna is used for GPS (GPS signal OnStar uses, not to be confused with a separate GPS antenna for integrated navigation; up front by the center speaker in VEs), cellular (for OnStar, not persona phones), and XM. It is not used for AM/FM. The AM/FM antenna is integrated into the rear window, just like the rear defroster is. This aligns with most other American cars.
The G8 came with the same VE Commodore rear window, therefore its AM/FM antenna stayed in the same place: on top of the roof. It also got redesigned to include Cellular (for OnStar), GPS (for OnStar) and XM. The VF Commodores still seem to use the whip antenna for AM/FM.
http://www.g8board.com/forums/4-tech-lounge/221953-g8-antenna-info-needed.html#post3059897
Some background, at some point, early 2000s, GM used three separate FAKRA connectors. Blue for GPS (for OnStar), Bordeaux violet for cellular (for OnStar), and curry yellow for XM. That's for a vehicle equipped with both OnStar and XM. If the vehicle was not optioned with XM, then the roof antenna would only have two connectors: blue (GPS for OnStar) and Bordeaux violet (cellular for OnStar). And even before those integrated antennas, some vehicles had a "wart" XM antenna up front and a standard OnStar antenna out back. Base models (no OnStar or XM) had nothing; again, AM/FM through the rear window.
They then combined GPS and cellular into one (Bordeaux violet) connector. Possibly because cellular frequency use expanded. Cellular in itself refers to the 850 MHz band. And GPS operates in the ~1,500 MHz band. Cell phone frequencies have expanded tremendously and GM's OnStar cellular provider (Verizon Wireless) has expanded their frequency use. So it would be safe to say that GM's newer antennas included a wide array of frequencies into a single antenna, hence combining the two into a single connector. This explains the 2014 Chevy SS antenna from Pace Performance that was linked.
The newer Chevy SS shark fin for 2015+ models (which looks better in my opinion) with two connectors (1 single curry yellow and 1 dual Bordeaux violet) and three wires was introduced because GM added OnStar LTE to the SS. The OnStar LTE provider is AT&T and due to LTE and its complex antenna design, it seems like it has been separated.
Summary: Removing the whip antenna will disable AM/FM on Australian VFs.