How do you remediate contaminated land?

depends on the type of contamination.  Usually it costs MORE to clean it than to leave it & run & disavow any knowledge of the land in question. Which many do until they get caught.  Then the fine is quite sizeable. 

.  Otherwise you get a bulldozer and scrape off all the bad stuff and load it in dump trucks and you "find a place that will take it LEGALLY" through much government red tape.  That is the hardest part because NO ONE WANTS CONTAMINATED dirt...as now that land would become useless because it is contaminated.   Then you have to buy clean dirt and truck that in.  

. Dirt is not cheap. Gravel is not cheap.

Talk to your city hall.  They are the ones to tell you how to do it seeing you are in Australia.   The rules do not really change ANYWHERE.