Yep, I've got this problem myself with my sisters 2015 GD SR Premium Hatch. Leaking into the left C pillar right into the spare wheel. I'd be looking down the road of a Section 54 of the Australian Consumer Law;

"Section 54 of the ACL determines "reasonable quality" for a product by taking into account various factors such as general use, price, labelling, etc. For example, a $5 t-shirt naturally has a lower degree of "reasonable quality" than that of a $50 t-shirt".

Hyundai are already dealing with Section 54 ACL due to their paint defects which has their cars (my sisters included) paint strip back to bare metal/etch primer. Hyundai are well aware of their paint issues and should be repairing vehicles as they are not of "Reasonable Quality".
This plastic cassette should also be included. For something so substantial to the vehicles' overall longevity to fail which is not only a safety concern but a massive concern for the interior which will eventually mould if not kept on top of, as well as the interior electrics that will fry. Hyundai Australia have absolutely no reason in my mind to refuse a replacement at no cost especially if the vehicle is under 10 years old.

How was everyone travelled so far with this thread?