See the Building a House thread, but your experience is well beyond that discussion. You seem to truly be a victim of the laws (and lack thereof) in regards to dodgy tradies and builders. Since Australia is often referenced as being owned and operated by property developers it will probably not change for the better any time soon.
For the general readership, this episode could be instructive in how bad it can get.
http://www.heraldsun.com.au/leader/north/incompetent-rangeview-estate-builder-david-brayer-fined-and-stripped-of-building-licence/news-story/125b1a3c53009eb3c129df19fb8f54ee?nk=cc9654097ce4b526ef42bad77ae7e5bb-1469864244
Compare the penalty with the probable costs involved for the home owners, and what it took to get attention.
