I can't help you with the species...sorry. Is that snow in the last photo? If so I would think that was the problem. As far as I know tree ferns here do not grow where it snows in Aust. They like it much warmer, some will take some sun, but most grow better with some shade...unless you have a mild summer. Wind will damage the fronds, they also need a bit of water through summer, at least every other day if it is hot and breezy. Some folk in the warmer areas here use a cool drink bottle with a small hole in the lid. Fill up the bottle and upend it in the crown. No advice for snow...sorry.

Better you should wait until the fern has developed some naked trunk (a few years for cooperi) so that the Tillandsia roots can have a solid medium to attach to. Otherwise, as the fern's scales drop off you're likely to lose any tenuous hold the roots may have since you'd be mounting directly on top of them. Does that make sense?

You mean Cyathea australis, yes? Without a working link to some good photographs there's nothing useful anybody can tell you - either about the apparently dying frond you describe - or about the overall status of the tree fern itself. There may be absolutely nothing wrong at all, but it's impossible to say without some visual information. Is the fern even rooted or just a bare trunk specimen? That's vitally important to know too. Nobody wants spots ON TOP of their leaflets, but you're not just making a fuss about the rows of spot-like sori on the UNDERSIDE of a sporing frond, by any chance...? One frond dying off is nothing to worry about in an otherwise healthy tree fern, especially in winter - C.australis isn't necessarily evergreen and defoliates entirely in many habitats - but some good pictures are going to be necessary to elicit the sort of advice that might set your mind at rest,

Hello, allow me to add a couple of points for those tree ferns that I so envy growing in your part of the world. Cyathea cooperi and brownii are a group of Cyatheas that require exposure to sun when they grow older. These belong to the Sphaeropteris group that is generally made up of fast and large growing tree ferns. When young, they need filtered shade. AND they need good drainage and constant moisture. 39C in the day? does the night temp drop to 20C or lower? If so, the Cyathea cooperi or brownii might be able to take a spot with sun. (Best is morning sun and afternoon shade) The other tree fern Cyathea australis on the other hand is a strictly forest shade creature.