If you weren't assuring us that it looked like a pansy earlier, I would certainly say that what happened is that your pansy seed didn't sprout and some weed seed did. We hear that all the time, people carefully nurturing something that sprouted where their seeds were planted, that turns out to be a weed.
Pansies have roundish to oval leaves:
https://c2.staticflickr.com/4/3161/2753 ... caf7_b.jpg
But I will take your word for it that this plant started out like that.
I'm sorry bird lover, but much as I love compost and frequently recommend it for other uses, I do not recommend compost for putting in flowerpots, especially small ones. It is too dense and moisture holding and tends to compact, excluding air circulation. And nitrogen is exactly the wrong thing. The trouble with this plant may well be too much Nitrogen. Whatever potting mix it is in may have added fertilizer that is too high in N. High N ferts lead to big leafy plants with no blooms. The pot is small for the size plant you now have.
My suggestion would be repot it into a larger pot. Use a potting mix with no added fertilizer and then add a bit of a high phosphorus fertilizer (that is the P in NPK). Flower booster fertilizers are something like 5-30-5.
Also note Aug is the wrong time for pansies to flower anyway--in the northern hemisphere. Pansies like cold weather. They are the first thing to bloom in late winter early spring. Mine usually die in the summer unless they are in a very moist shady spot. If you can get it to survive the summer, it will bloom more in the fall when the weather cools. You are in Australia, does that mean you are going in to spring now? Then definitely proceed as above.
Best Wishes and let us know if you get flowers that way.