Train to Hong Kong and back. Does 72 hour visa cover re-entry into Guangzhou?

Hi, I'm looking to fly from Brisbane to Kathmandu via Guangzhou. On the return leg I was thinking of catching the train from Guangzhou to Hong Kong and back and stay in HK for a couple of days. If the time between landing and departure in Guanzhou is less than 72 hours can I use the 72 hour free transit to return to China from Hong Kong? As I'm from Australia I believe I won't need one to enter HK from China either. Is that correct?

Thanks,
David.