Indoors air is usually quite dry and quite still. Having the plant in the direct sun behind glass is a bit harsh for it. And obviously there's not enough water. Put simply, the water in a plant 'inflates" the cells and causes rigidity (or technically speaking turgidity). Particularly in the new growth this keeps the plant upright. With a shortage of water the cells become deflated and sag. Provided you've got good drainage you need to increase the watering.

By the way, there's more than a dozen "Australian Treeferns", Cyathea cooperi is just one of them.

Hi
I've found them very difficult to grow in a pot particularly in the house . if you haven't repotted ,probably low humidity and not enough water . Is it the new fronds that are wilting?? i find that I must water the entire plant or new or parts of new fronds won't unfurl.
I now grow them outdoors in a specific bed and they are doing well there Good luck gary

It is clearly a Cyathea cooperi, as label says.
Drenching it some more in the shower seems to solved the problem.
This plant is a primo candidate for me to overwater now that I saw how thirsty it was :)

Does it get any easier maintaining it once it gets bigger ?
I'd imagine it stores more water.

hi
I find they need even more water as they grow. They also get very large very fast lol Mine went from under a foot to over 8 feet accross in a bit over a year yet still has less than s foot of trunk!! gary

I can start noticing how fast it is growing.
Second "leaf" is already starting to unfurl. It did not even exist 2 weeks ago when I bought it.

Seems like I need to give it a serious drench in the shower about every 3 days. Before the soil seems dry on the top even.

IF IT IS DRYING OUT VERY QUICK IT PROBABLY NEEDS TO GO INTO A BIGGER POT, THEY ARE A very vigorous grower and roots soon fill a pot there soon use up what moisture in the potting mix. we grow them in the ground here in aust, growing indoors I would suggest also a self watering pot, or some type of reservoir for it. Denise