Yes the burden of proof is on CASA that you took of at 42C, but that would be easy to prove, and as it's a clause of strict liability you are now liable.
Easy to prove?
There is no law that says you can't take off at 42C. The only applicable law says you can't take off
overloaded.
But how can CASA prove you were overloaded when the charts only go up to 40C?