To rug or not to rug?

Trust me, he won't freeze!!! Just throw him out a nice big pile of hay and he'll be happy as larry!

Im just worried because now he is shaking a tad :/ Im sorry for loosing my marbles, but he is my partner in crime and I love him to bits

Not much more we can suggest unfortunately! There's only so much you can do if you can't catch him. Throw him a bunch of hay, or try harder to get him - shake a bucket of grain or something and get a rope over his neck. If he's being stupid though, its likely he'll keep himself warm by running around anyway.

HAHAHAHAHAHAHAHA!!!!!!!! 73degrees? Seriously? It's like 102degrees outside here right now. Lucky! Our winters drop to -10, and our horses don't get blankets til 20-30degrees. (Average paints/QHs).
Anywho... the only reason to really rug a horse is if they're in danger of letting their body temp. Drop too far. I'm pretty sure 73 isn't nearly cold enough. Try 30degrees. Yeah, he might shiver a bit, but horses can shiver for a whole day with no side effects. It's how they function.
Besides, he's got a coat. He can handle a bit of rain.

Don't laugh at me Rascalboy :P Where I live 23C (73F) is cold! He seems to be alright today and is standing out in the rain. Hopefully tomorrow we get a break from this wet weather :/ Thank you everyone :)

I have to say that 8 celcius (46 F) is a warm winter day for us, quite warm. But, then I would suffer in your heat, for sure.
The horses at our place are never rugged. But, during winter, they get a lot of free choice hay. They have trees to get under. We only worry if it's windy and below -8. As long as they have a lot of hay and wind breaks, they are fine.

Do horses in Australia grow long winter coats?

Unfortunately, not really in my experience Tiny (not talking ponies and drafts here). Our winter is so mild and starts so late that they really don't develop a proper one - which would be fine except we do get cold snaps coming up from the south and the horses aren't prepared for them.

I do think many people over-rug in Australia (two flannel rugs in our 35-40C summers is ridiculous, I don't care how much judges want a nice thin coat), but as rain prevents a horse from being able to lift the hairs and create a layer of warm air for insulation (if he's used to steamy QLD weather he may find a wet 23C quite cold!), but if it were me I would at least try and put a rainsheet on this fellow to keep him dry.

I remember once seeing in a horse care book a net sheet that was to be used under a light rug or rainsheet, the idea being that would allow a layer of warm air to form between the coat and the rug - has anyone ever seen these IRL? The concept works, I used to wear a net crochet top under a thin skivvy when out in the early morning air and it kept me nice and warm without being too hot.

I'm right down the bottom, in South Australia and it can get pretty chilly down here - nothing like the US with your snow though!
Our horses do grow significant winter coats, particularly if they are in the paddock unrugged.
Spighi had a very heavy winter coat by April this year, so I needed to clip him early, by the end of April.

I did manage to catch him today and I have put a light rug on to protect from the wind and rain :) And what I'm about to say will make you all laugh... In regards to the question about winter coats, Freddie has the thickest winter coat I have ever seen on him this year! He is like a sheep! It's mainly just his belly, legs and neck however. I know a girl who owns a 23 year old stock horse who has a very thick coat all year round! Poor guy suffers in our usual 90%+ humidity unless he is shaved

My horse is never rugged, even when it drops lower than -30 by Celsium (which is rather common here in winters), and he deals quite nicely. This was also his first winter in a 24/7 pasture, no stalling at all, and he showed no interest in the shelters, having the nice, fuzzy winter coat of his. That's just an example on how well horses can adjust to different weather, if they have the opportunity. The key to warming horses' body is free range quality hay (or grass, I believe you have grass at 23C) and letting their termoregulation to figure it out by themselves.