G'day Twr, it's not illegal, but, if the truck is displaying a "Do not overtake turning vehicle" sign the silly prat trying to do so is held responsible no matter which side of the truck they are on. If the truck isn't displaying the sign then the truck driver is responsible.
No need to point out how popular the sign is down under.
It must be of regulation size, displayed on both sides of the truck, reflective etc. and care taken by the driver of the truck. If the silly prat stops part way the truck driver can't drag the trailer over their bonnet, and giggle at them. They must give them chance to reverse out.
Bit of a grey area but in general the truck displaying the sign has "right of way".