Chinese free transit - 72hrs

Travelling on an Australian passport, I plan to fly Pyongyang to Beijing - arriving 10am on a Monday.
Then travelling back to Melbourne Australia, later in the week.
Would you know if a 72hr free transit would officially begin from my arrival, 10am that day? If so, I should get a flight back to Australia, departing Beijing before 10am on Thursday.
If that flight back to Australia, stops at Guangzhou or another internal city, before landing in Melbourne, would you know if that voids a free transit obtained for Beijing?