I've been looking at my "heath" photos (still sorting to upload) and I have some with similar shapes at the end of the flowering season, as the flowers brown off. Mine aren't quite so "ball" like, still have the petals intact and it's not the same species based on the leaves but certainly similar.
I’m confident in saying that these are not fruits. Lissanthe strigosa is commonly called Peach Heath for its peach-like fruits. They are about 3 mm in diameter but have a coloration like an unripened peach. Most tellingly, however, the fruit is covered by a downy bloom, just like the bloom that covers a peach. What we have in this photo is most likely some sort of insect-mediated damage, possibly a gall-like process.
It had occurred to me to ask Roger or Kim, but Betty's assessment that it is a virus seems very likely. So I would rather not bother them with this question.
I had an unexpected meeting with Rainer Rehwinkel at the site. The plant was easier to find than I expected and he assured me these unusual growths were galls. See the following sighting: https://canberra.naturemapr.org/Community/Sightings/Details/3406940 We also noticed that other specimens of Lissanthe strigosa nearby had galls as well, but only one or a few, not the heavy infestation that this plant had. But it indicates that these galls are common in the local population at least.
It was conclusively a case of galls, because each of the swellings had a minute hole in its side, where an insect, possibly a wasp, had emerged following its larval and pupal stages spent within the swelling.