Which is my point really, the US and the UK didn't decide to share submarine nuclear propulsion technology (one of the most closely held secret military technologies in the world) along with Tomahawk cruise missile technology with Australia out of the goodness of their hearts. Call me cynical, but I don't think the US and the UK do 'favors' like this unless there is something substantial in it for them as well.
I suspect your post earlier is closer to the truth, Northern Australia provides multiple basing opportunities for both strategic aircraft and Naval vessels, that are only slightly further away from mainland China and the Taiwan Strait than Guam, are out of current PLA IRBM range, but aren't as distant as Diego Garcia in the middle of the Indian Ocean.