Thanks for all your replies. Sorry for the delay in my response and sparse information in the initial post. Yes, it is now too cold to germinate them, I think.
The seed should be fairly new - I ordered it early this year. Maybe the post-harvest ripening thing could be it, because this link says there's a 'dormancy period' after the seeds are harvested? https://www.anbg.gov.au/gnp/interns-2004/themeda-triandra.html "Seed may be sown straight away with some success in mid summer to autumn but generally the seed has a dormancy period of up to 12 months depending on the area where it was collected."
I had read a journal article which said it's better to cover them with soil than just place them on top, even though they are a type that drills into the soil when moisture is present. I also didn't want them blowing away or being eaten by birds. The mob I bought them from recommended using a capillary / wicking bed where you get a large tub, put sand and water in the bottom and then sow individual seeds in forestry tubes poked into the wet sand. I didn't have the necessary materials at the time so I just tried sowing them in various spots around the garden and kept the soil moist. I also tried covering some of the seeds with coco coir instead of garden soil, which held moisture better. None of them germinated which was disappointing as I know they don't like being transplanted. Yes I'm trying to foster biodiversity in my garden and plant native and indigenous species.
When it warms up I'll try the capillary bed. This link talks about smoke water but that could be tricky to make http://anpsa.org.au/seed.html
I became frustrated because I really want this species in my garden and initially I read that they were easy to germinate. And I have now wasted about half of the seeds I bought and feeling guilty because I keep reading how the seed supplies are limited. Maybe more time is what's needed for it to work.