I lived in Sydney for 6 years, food and other prices are more or less the same as in the UK. There are, as mentioned, plenty of cheap food halls to eat in , with a far wider variety of ethnic foods than you find in Britain. In Sydney, visit Chinatown, its small but brilliant. Loads of good food at cheap prices. Also lebanese, philipino, nepalese, all in the central area of Sydeny.
Other main cities like Melbourne, Adelaide, Brisbane, Perth are much the same.
Australia is one of the best countries in the world to live in, I'm sure you'll love it.
Don't forget youe sunnies (sunglasses) thongs (flipflops) & budgies (swimming costume!)