Transit 72 hour

I am flying from Melbourne to Beijing - landing in Beijing on 15 June at 11.05pm and then departing Beijing on 19 June 1.35am. By my calculation this is approximately 74 hours. Assuming I will be in the airport for an hour when I land and at least two hours before I depart, am I able to use the 72 hour free transit? Does it cease to apply once I re-enter the airport?