The ideal change to this 457 rort is to mandate that for every 457 used (or even for every two used, say, just to soften the economics a little bit), requires one Tafe or University Graduate, Paid Intern, who is an Australian Citizen to be employed as a trainee at the same time.
The idea behind 457's is supposedly to support industries that have a skill shortage. Those industries should be putting just as much effort into removing said skills shortage themselves. When there is mutual obligation behind the use of 457's, watch their usage rapidly go back to what it was meant to be.