If it's water and not coolant, check the evaporator drain hose. I'm not sure in Commodores, but in pretty much every other car, water leaking into the passenger footwell means an evap drain. The water is coming from the evaporator operating (which is normal) which when the hose is clogged, gets flooded and leaks. Water should come out from somewhere under the car when you have the aircon on, if it doesn't, there's another clue. It'll probably be a lot cheaper than throwing money at the heater core to check this first.
P.S. I just spotted the overheating bit. That makes me less optimistic, but it's still worth checking to see if the aircon drains water, I guess.