Bush is the forest, or rural/outback places, and tucker is food. So bush tucker is food you get out of the bush, like native berries, native bananas, orchids, leaves, possums, and witchetty grubs. It is traditional diet of aborigines.
Billy is a tin with a wire handle usually used to boil water and tea in over a fire. It's always handy to have one with you because you can use it for all sorts of things, like catching bush tucker which is trying to get away from you. Billy tea you make by boiling the water then throwing in the tealeaves and gum leaves if you like, and letting it come to the boil again. You then take the billy off the fire carefully with the woollen sleeve of your shirt. Then you swing the billy right round upside down a few times. You pour the tea into enamel pannikins and add lots of sugar and give your dog a big roo bone while you sit on a rock and contemplate. They used to use teatree leaves for tea, but I don't know what this tastes like..I suppose you can call it manuka tea.
Chockers is short for chock-a-block, or filled right up, like you are after you have 3 or 4 vegemite sangers..
Little John, you never told us how Brendett likes vegemite. Have you been fighting over that jar?