Hi, I'm in Tasmania and it is reaching 5 degrees C here (as you'd know seeing the news!) but it never snows like Melbourne and Sydney recently have. Sorry I don't think i read where you are. . .Brisbane
Anyway wherever you are in Australia, its probably best to have him indoors overnight when you can, but constantly taking him in and out would be a hazard (warm/cold).
I have finches in a small aviary like you explained (is yours like one of those at
www.animaltuckerbox.com.au) anyway its also aluminium and most aviculturists have warned about keeping birds in these cold aviaries. So i moved mine into the patio and none have since died.
Tassie is supposed to be cold, but it doesn't ever get snow or extremes of temperatures. Snow would kill a bird in an aluminium aviary for sure.
A young bird outside in an aviary needs something warm, aviary birds normally have other birds to snuggle to and nests.