can i get 72 hours free from Nanjing?

Hi, I am travelling to Hong Kong from Sydney through Nanjing. I have a 12 hour layover in Nanjing prior to my flight the next day to Hong Kong. Do I need to obtain a visa prior to travel for this 12 hour stop or is it just easier applying for a temp exit permit at the airport? If so does anyone know the cost for this permit. On my return I am flying through Shanghai.