Eggplants can be sensitive to temperature. If it's too hot, they may not set fruit, probably because of sterile pollen (from the heat) as with tomatoes. My Diamond eggplants and another unknown variety wouldn't set in the heat (despite numerous flowers that were not in the habit of dropping particularly), but they would set fruit when it cooled down. However, if it cooled too much they wouldn't set fruit then, either. If you live in or near the desert you'll likely have a particularly hard time because of the major shifts in temperature between day and night.
I propose planting cold-tolerant and heat-tolerant eggplant varieties. Don't plant them in the shade. Apple Green is cold-tolerant. Aswad is heat-tolerant. Malaysian Dark Red might be both. Locally adapted varieties are important, in my opinion. Saving seeds every year rather than buying new ones from somewhere else may help them to adapt more.
Looking at what the Internet says about Brisbane, I'm guessing if you have temperature issues they would be with it being too cool at night. I don't know how cold it gets at night, though.
See this Q/A, which is related to your question and my answer.
It's also quite possible that there's an issue with your soil. I know you're not talking about tomatoes, but they are closely related: I grew the same varieties of tomatoes as my neighbor (from the same seed packets, even). The only notable difference in growing conditions was the soil. My neighbor's tomatoes grew huge plants fast with a whole bunch of flowers (but the flowers wouldn't set fruit for a long time, it seemed). My plants were much smaller and had fewer flowers, but had more fruit earlier on. I'm guessing my neighbor's soil had more nitrogen and mine had more potassium (I gave mine a fair amount of potassium sulfate, rock phosphate and basalt rockdust). I don't think my neighbor amended her soil. Last year she had plenty of hay about; so, that may have left some nitrogen in the soil. If it weren't for the heat, I'm guessing her numerous flowers would have set fruit much earlier. Potassium does provide some heat-tolerance, I've read. Phosphorus helps with cold-tolerance.