Ah, that is for the slow roasting of drop bears.
No seriously, I all depends and is hard to say without seeing the building including its substructure before it became a ruin. Debris can make it hard to tell what the real floor structure and what was underneath originally.
Some were to slowly radiate heat into the floor for heat and to keep the flooring dry, many had cob floors- mud basically. It also helps in keeping the moisture seeping through the soil from around and under the building. Same goes for those built on piers with a wooden floor, it keeps under the floor dry and warms it slowly.
That's a educated guess.
Another possible reason is to ensure the cool cellar in the floor does not freeze in winter. Many old homes had a cool cellar in the floor in the kitchen or close to the fire. It helped regulate the temp for the food stores and provide a low moisture environment.
I have seen this setup before.
I would expect it is a throw back from the English times exported to Australia and adapted. The building of fire systems with dual use and even pumped water/air heating goes all the way back to the Romans who were experts and refined it to high art.
I will look into further and see what I can find.