Hello Feliz -- I hope the following helps:
(1) Australian wheat is pretty hard (ie, high protein/gluten), and so recipes have always adapted or been designed to cope with this. Self-raising flour is
not cake flour! It is usually 10+% protein, while cake flour is lower, it is not treated in the way that US cake flour is... and most importantly, it contains a number of raising agents, so would not be suitable.
(2) The standard Australian solution to the cake flour issue is to mix plain (all purpose) flour and some cornflour (cornstarch). Stephanie Alexander uses a ratio of just under 1 part cornflour to 3 parts plain flour. So, if your recipe calls for, say, 240gm cake flour, then combine 60gm cornflour and 180gm plain flour. Other books use a smaller proportion of cornflour.
(3) If the above approach isn't satisfactory, it's possible to buy some soft Italian flours at various European shops (at least in MEL and SYD), and there's a USAmerican shop in MEL which sells "Swan's Cake Flour":
www.usafoods.com.au