I’ve read a lot about running Diesel vehicles on plant oil, but one thing that was never clear was why some people claim that you need special chemical additives.
The article about converting vehicles to vegetable oil on the VegieCars.com  site explains all this. It seems that if you want to produce fuel which can be used in unmodified vehicles then you need to add a mixture of methanol caustic soda. This is going to be difficult, dangerous, have some expense, and probably not be that good for the environment.
The other option (which they recommend) is to modify the vehicle to accept straight vegetable oil. This means pre-heating the oil before it enters the engine (to lower it’s viscosity and make it vaporise more easily) and to filter the oil to remove solid objects and water.
A possibility is to have two separate fuel tanks so that you can switch between plant oil and petroleum based Diesel fuel. This is an option if driving in a cold climate (probably not an issue in Australia apart from a few mountains) and if you are concerned about the quality of your plant oil (a bad batch could clog the filters and force you to use petroleum based fuel).
They also have an interesting cost-comparison page to show you how much money you might save by using plant oil .
Their site is very interesting and has some good technical information, even if you never plan to drive a Diesel vehicle it’s worth reading if you are interested in cars.