Australian citizen, 77 hours in Guangzhou, visa exemption ?

Hi, I am an Australian passport holder, travelling in August from:
Melbourne - Guangzhou (3 hours transit only) - London,
London - Guangzhou ( 77 hours stay ) - Melbourne .

I will be in Guangzhou for a little over 3 days, based on scheduled flight arrival and departure times - actually 77 hours in total.
Am I able to enjoy a 144 hour visa exemption ? How strict is the 72 hour visa exemption ?

Thanks :-)
Matthew