Australia is not cheap. Going out to eat at a café or restaurant is expensive and so is the grocery bill at the end of each week. There are two major supermarkets chains (Wooloworths and Coles) and their prices are not that competitive. I usually shop at both as each have different things that appeal to me.
But if you want to buy good quality fruit and vegetables at very appealing prices you should definitely head to the Brisbane Markets, in Rocklea, on Saturday mornings. You can also buy fresh bread, cupcakes, speciality sauces, nuts, eggs, cured and fresh meats, etc.
There are also plenty of places to have coffee and snacks. I even found a stall of Dutch poffertjes which reminded me of the time I spent in The Netherlands. In these stalls you can also find some very interesting international foods so enjoy a nice breakfast, brunch or lunch ;-p
Across the street you can find the Flower Market, which is open everyday. If you want to buy someone a nice gift this is the place to go! There are many options and though flowers are quite expensive you can find nice arrangements for a reasonable price. I bought a nice orchid which is still sitting pretty by the window.