I am curious what the weather is like in your area. Sadly, I have never been to Australia (gotta do that one of these days soon) But my understanding is that though it gets cold for a few hours during the night in winter in your area, it isn't so cold during the day, even in midwinter. So, if it's cold for only a few hours, it isn't the end of the world if they feel coldness for a few hours.
We don't rug our horses at all where they are kept. They are out 24/7 with only a runin for shelter from the wind and rain. I used to worry about them and think how cold they must be. Then when I'd go see them in the morning, they would just be standing around as usual , even with snow on there fetlocks or rumps. They weren't shivering. It's amazing how sturdy they are.
Howver, I know some breedsd don't handle the cold well, or will go through a transition time.
I find it surprising that folks in Australia over rug their horses . I have this stereotype in my mind of Aussies as being really rugged folks who require their animals to be equally as rugged and disdain fancy-schmancy horse stuff.
P.S. I went for a vigourous trail ride of 3 plus hours up and down, trot and canter and gallop and the horses are all covered with long hair now. Of course they sweated, but that's how they cool off and they were fine with a quick towelling and a short cool down walk , into the trailer and home. Mind you, it wasn't THAT cold out (about 48 f)