72 hour transit permit

hi,
thanx for the article and responses to others. i am planning on a trip to Africa via Beijing from Australia. I am getting 2 tickets . one return ticket from sydney to beijing and another return from beijing to Africa. With my flights, i will be leaving sydney to beijing, and then catch my flight from beijing to Africa. But when coming back from Africa to Beijing. is it possible to use the 72 hour permit before continuing to Sydney ? if so do i inform the airline which brought me from Africa or to the airline am going to take back to sydney from Beijing?