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.

If only you guys could see the back of Lumahai Valley in person, you would be astounded and alarmed at just how many there are! These ferns are bad- on par with miconia if not worse because they are spread by spore. The future forests of Kauai and probably all the islands will have no shortage of these ferns. Sadly, they can form near monotypic stands where the massive amounds of leaf litter from fallen fronds smother most understory life and create much debris (and far more than the Hapu'u). Perhaps if they cut most, if not all, the tree ferns down in peoples Kona yards, they could buy a few decades. I have seen them popping up on the Hilo side even in remote areas. Once I noticed one while in the Hapu'u forests. You can kill them by dicing up the growing top. Make sure you chop down until you get in the core. I disagree with feral ungulates carrying spores (The spores are everywhere in the air) but do notice that open dirt, as found in pig diggings, under houses, and in yards raked to the dirt, tend to have more seedlings coming in. They are extremely- and I repeat- extremely shade tolerant. I have them growing under my house among other places. They are easily distinguished from Hapu'u once you get the eye for it.

Yes you can cut a Cyathea cooperii off if it gets too tall. Keep the top plus enough of the top trunk to replant at the height you want plus half a metre to be buried in soil. . The bottom part will not regenerate and needs to be removed. Trim back the old fronds being carefull not to damage the new unfurling ones. Replant in fresh slightly acid soil, give it some organic fertilizer, water the trunk and base well but not the fronds. Make sure there is adequate drainage. give the plant some dappled shade if you can.

I live about as far away from you as possible so my advice may mean nothing - to me it sounds like they are in shock. It could be stronger winds then they have ever experienced before, stronger sunlight or something about the weather that they are not used to dealing with (like salt spray). With mine, I water the trunk every day. I mean I spray the trunks of all the tree ferns more than I ever water the soil in the pots they are growing in. They get most of the soil moisture from rain but the wind can dry out the trunks which seem to prefer being moist all the time. Whenever I repot mine I cut off all but one or two fronds to help it adjust to the new soil and any damage done to the roots while being repotted. I've had mine for many years and the tallest one is over 6 feet tall. I don't really baby them, but I keep them moist, water the trunks and try to keep them out of the wind.