There is a simple way to stop anyone who is thinking of circumventing immigration laws, but it would require CASA to do something useful for a change.
Simply link issue of CPL and higher to appropriate residency/work permit status. Many countries do this. Or do what the Yanks used to do (probably still do) and endorse the licence 'not valid for employment within the USA or its territories'. It allowed a pilot to ferry a N registered aircraft to or from the USA and even across the USA, but he could not be paid locally.
Any operator wishing to abuse that limitation would have to go to a lot of trouble setting up external bank accounts etc. Imagined savings compared with employing, training and paying locals the award wouldn't be worth the hassle.
Fire fighting using heavy aircraft is a different story altogether. Very specialised work, and in any case the aircraft are not VH registered.