Some of the new diesel cars come with a separate Diesel Exhaust Fluid (DEF) tank.
Several commercial DEF products are available, AdBlue being the most popular brand (ISO 22241)
I found a 10 lit container at Woolies Caltex for $40 which is significantly cheaper than what any dealership sells it for
However, some Metro stations have an AdBlue pump and sell it for 60 cents a litre. All you need is to bring in your own HDPE container
Adblue Diesel Exhaust Fluid