Aussies..What kind of lizard could this be....?

I live in suburbia Sydney, no bush etc., however I have spotted this large looking lizard a couple of times over the past few months in my backyard. It is living in behind my shed, where there are overgrown bushes. It is not a blue tongue or frilled neck - in fact it has quite a distinctive look of its own. Its quite large (maybe 35-40cm long) four legs, but has a distinct tail. Rather than its tail petering off to a long thin one, its quite large and stumpy. The best way I can describe it, is that its the shape of a long thick cactus (stump or penis) I have searched google sites but cant seem to find one that esembles a tail like this - so weird - I've never seen anything like it. Does anyone know what it could be..??