[crayman]

2012 GD, gave it a wash on the weekend and it looks like the hatch is leaking somewhere.
You can hear water slosh about as the door is raised/lowered and when it's fully up, water pisses out of a vent over the items in the boot, very nice, just like the old Commodore.
Can't see any drains that might be blocked and I presume the water is getting in via the tail light assembly?
Back to Hyundai, again.

[AlanHo]

I had a similar problem - and then realised my rear window washer wasn't working. The plastic feed pipe had come detached inside the hatch. But in my case the water was a much smaller quantity than you mention.