I believe a big motivation to cull crocs would be safety of humans at our famous Australian beaches.
It has taken quite a lot of years for saltwater croc population to recover from the 70's. Now the crocs have been extending their territory further and further down the east coast, it is just a matter of time before we start losing humans to crocs.
Bear in mind, crocs can live a long time. And they keep growing throughout their lives.
Soon, some management will have to be implemented.
Just think how popular something like Kris, the 8.5mtr Normanton croc, would be if he appears at tourist filled beach!