Our question is - do we need a one entry or a two entry visa.?

My wife and I will be travelling to China, landing in Shanghai We will be in China for 10 days, then we will travel to Hong Kong for 4 days. We will fly back to Shanghai from Hong Kong then catch the flight back to Australia that day. We will not leave the airport, we will probably remain in transit till we board our flight back to Australia.
Xie Xie
Peter and Jenny