Visit Shenzhen /Xiamen from Hong Kong

Hi I am Australian, travel from Sydney - Paris - Sydney with Cathay Pacific. On the way back from Paris, on arrival in Hong Kong, I would take another CX flight to Taiwan for 6 days; then go back to Hong Kong for a week. During stay in Hong Kong, I would like to visit Shenzhen or Xiamen for a day or two. Do I need visa to visit Shenzhen or Xiamen from Hong Kong? Thank you for your info.