Probably not that high, but remember you have to claim alot of things on those Centerlink forms you regularly might have to re-do. Centerlink might cross-check with your bank account transactions with the Taxation Office and if they find out you've been getting paid like you would be but not getting taxed.. well, you'll be in for some flak.
The best deal for avoiding this shit is if you get them to just pay you in your hands, once a week or whatever at somewhere not public and that it's just a deal between you and your employer... no one else hears or knows about it.