But how can CASA prove you were overloaded when the charts only go up to 40C?
FGD135, there is no requirement for the Prosecutor (which is not CASA) to prove you were overweight. It is up you to prove that you were not.
The law clearly (albeit in my opinion, stupidly) states that you must not allow the aircraft to takeoff overweight.
If the charts cannot be extrapolated and you are operating outside of a limit of that chart, then you are breaking the law. The law is one of strict liability that permits no defence.
Sadly there is nothing to stop CASA ramp checking anyone who commits aviation on a +40C + SL day and asking how you complied with the strict requirement. The short answer is you can't and they could, if they wished, send you the 'fax' and refer to the prosecutor.