I have found some at the Baby & Kids Markets (google them, they have monthly markets in a few locations around perth). It's mostly kids stuff, but some people do sell maternity also - I got 2 tops & jeans for $15.
A friend went to the salvos and just bought bigger sized stuff. I also found a lot of long, stretchy and/or very tent-like tops at cotton on.
I work from home so hobo works for me