You are in your winter. There are a bunch of Aussies on BYC. I suggest you start a gardening thread for those of you in the Southern Hemisphere. Or perhaps go to your Australia thread and talk to the folks there for advice. (look in the Social, where are you, where am I! thread).
Feed the soil, and the plants will feed you. Your garden also needs lots of sun. Full sun is best, decreased competition from weeds, and appropriate water (and temps) If you have tree roots encroaching on your garden space, they will rob nutrients. It's amazing how very far roots can travel.
You might want to start by having a soil test done. If nutrient balances are off, your garden will fail.