I suspect Google uses their own projection for maps in Australia, as they do in the US, which, depending on how they defined and implemented it, could/should ad

I suspect Google uses their own projection for maps in Australia, as they do in the US, which, depending on how they defined and implemented it, could/should add the intermediate layer needed to help resolve this quickly. But there are way too many variables to speculate in any meaningful way. Depending on how they store imported data vs different Google generated data, you could be talking about a simple transformation parameter change to their projection, or you could be taking about some complex, varying corrections. For instance, if Google was already using street view or proprietary aerial images for ground truthing, they may have already compensated for the shift in various data sets, in different amounts, depending on when it was collected, and how they handled location adjustments due to discrepancies. But being Google maps, I wouldn't expect accuracy better than a few meters for any given dataset anyway, given past performance.