Does anyone know if this is a parasitic plant or an insect infestation?

It is quite similar to insect galls produced by Dicranoses congregatella (Lepidoptera) on Schinus species here in central Argentina

Without knowing about Pilostyles,I would have said that they're galls. But now I'm not so sure. I can see holes in some of the "galls", which may mean they are exit holes,, but they may be the remains of flowers. Have you tried dissecting them under the microscope? If they're flowers you may be able to see a few sepals or petals still attached.

Dear Dr. Kristenhusz, I am not working in Australia but in France. I do'nt know name of the plant on which are the organisms suspected to be either one gall insect or plant desease. For me as comparing with some deseases occuring on European wild or cultivated plants, they are perithecia of a plant dissease (fungal desease). But I have forgotten my cursus on plant desaeses ! I cannot say what is the name ot this fungal desease because I do'nt know name of the plant stem. Sincerely yours.

Dear Dr. Kristenhucz. This does help: fruiting Pilostyles parasiting an individual of Schinuus longifolius. Both species are very common in Uruguay. Best wishes.