As ChRiDDa said the hole in the pollen filter box will be blocked and the water enters through the pollen filter and into cabin as it is the only way for it to go.
Remove cowl below passenger windscreen above firewall, look inside box and you should see a tide mark and some water in bottom.
Remove drainage tube beneath hole and flush it out or what I did was poked a piece of wire through to unblock it. Then clean any leaves out and dirt with a rag then reassemble
, this happened on my sons VE Omega and I thought I had fixed it with cleaning the drain tube, the next heavy rain it leaked again. I did some searching on the Pontiac G8 forum and this is a common problem with the VE Commodore and Pontiac G8.It can leak between the top of the pollen filter box and firewall... I Just applied some Sikalfex along the seam and let it dry reassembled and it hasn't leaked since
So either one of these options should fix it