In reply to Tya's comments very interesting. Though the comment about needing a Hong Kong pass i am not sure. As my gf does not need her HK pass if she has a flight out of the airport. Also i have been with her at the Chinese side and i can see they ask for 2 things, a look at the itinerary and her Chinese passport. This is at the crossing between Shenzhen and hk but one would imagine its not that different for flying into hk.

HK allows Chinese up to 7 days in HK before hand if they are going to fly out of the airport.