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,

The more I try and read between the lines, the more I suspect there's something catastrophically wrong about the growing conditions here. Assuming moisture levels really ARE adequate (dehydration is the usual cause of tree fern failure in gardens) I'm wondering if you've misapplied fertilizer or some other chemical treatment? A neutral or slightly ericaceous compost mix, high in organic material (gravel and rocks being inherently unsuitable), is all that ground-planted tree ferns require - they hate inorganic treatments, chemical salts, and strong nutrient, concentrated tomato feed in particular. And using any kind of additive to 'de-stress' it sounds crazy!

When ferns react badly to your planting regime, you've got to STOP doing what you're doing...not do more of it. I think a lot of people simply up the dosage!