I am size 18 too - I got some fabulous stretchy but flattering work pants and 3/4 pants from Myer in the Ladies BIB (Big is Beautiful?) section, nice wide waistband that is super comfortable. Loved 'em so much I went back and got more and have practically lived in them this pregnancy

Kmart have some lovely cheap maternity tops too - not a huge range and the size 18's seem to sell quickly, but I have a few staple pieces from there too