Clay soil is usually really high in nutrients, its just that the nutrients can't be used. THe best way to improve clay soil is to get yourself a few bags of gypsum (or if you know where a house is being built you can use the gyprock off cuts, but gypsum is so cheap its worth buying it in bags.) Dig the gypsum into the soil, this is usually pretty hard as clay soil is very hard to turn over, then scatter it over the top of the grow site and water it in. Because this takes a while to work you should make a raised bed. Because you have to lug all your materials into the bush the lightest way to build a bed is as follows.
Get some hay bales, cheap straw is good and very light. Put 2 hail bales next to each other 60cm apart. Then get another 2 hay bales and put these at the ends. Get some bailing twine and tie this around the outside of the bales so that when you put dirt in the middle the bales don't move. Put a 10cm layer of sand/gravel at the bottom of your new bed and then fill the rest with nice friable soil. Make sure you put the new bed on top of the area where the clay soil has been treated with gypsum so that by the time the roots have reached down there the soil will be improved and will relaease all those locked up nutes. Also if the treated soil is kept moist it means that next year you won't even need to build a raised bed, you'll be able to grow straight in the ground.
If you are in a dry area line the inside of the hay bales with newspaper to prevent water loss to evaporation.
Hay bales are such versatile things. I had a mate who got some of those hay bales that come wrapped in plastic. He cut a round hole in the top of each bale and rigged up a run to waste hydroponic system outside. In the end it was really hard to keep up the water but he pulled it off and the buds were amazing! The potency of hydro with the yield of bush!