Do I need Visa if I fly China Southern Airlines from Australia to U.S via Guangzhou (stop < 48hrs)?

I am using Australia passport to travel flying Southern Airlines from Melbourne to L.A stopping in Guangzhou for about 4 hours. 4 hours waiting in the Guangzhou airport no problem, but on our way back to Melbourne from L.A., we arrive in Guangzhou airport in the morning and back to the plane flying back to Melourne in the next day evening, if we want to go out of the airport, do we need Chinese visa?

Thanks for anyone answering.