Hi,
I don't drive a Sunny, but I suspect that your case has nothing to
do with the brand/model of the car at all. Here are some points to
consider:
(1) Fuel tank gauges are never totally accurate. There is a
flotation device in it which measures the level of petrol in the
tank....and the flotation device (or its supports) may be faulty.
Its just a simple mechanism which is subject to failure....just
like everything else. And I believe that the fuel tank gauge is
something that is not fully tested before shipment. This may be
your case.
(2) For fuel injected engines, there is an oxygen (O2) sensor on
the exhaust side of the engine. It senses the amount of unburnt
oxygen and tunes the petrol feed accordingly. If the O2 sensor is
faulty, it may cause the system to inject more fuel. Signs of this
can be vibrations when idling, etc. But in your case, its so
extreme that you would be able to smell fresh petrol coming out of
the exhaust! Besides, judging from the amount of petrol lost, its
probably beyond your ECU's programming. So this is unlikely your
case, I think.
(3) Your car hasn't yet passed its break-in period. The piston
rings, etc are still tight and there is lots of abnormal friction
to overcome. Cars usually show marked improvement after the first
10,000 km. But then again, your case is so extreme that this is
unlikely the case.
(4) Petrol tank leaking, tank cap not in place, etc. I hope this is
not the case. 
Hope you're not a smoker
too.....
(5) The ECU (the micro-controller circuit, the "brains" of the car)
may have gone crazy too, injecting too much fuel. Then again, your
case is so extreme that you would be able to smell the unburnt fuel
in the exhaust. So maybe this is not the case.
(6) Fuel injection pump spoiled or leaking. Maybe there is a
permanently opened nozzle or something....and the fuel is just
passing through freely. Never heard of this before....its something
I just came up with 
. Don't think its your case lah.
The good news is....your car is new and under warranty. So instead
of guessing, its best to bring it back to Tan Chong IMMEDIATELY for
inspection.
