Sydney to Tokyo

Both your itineraries are eligible for 24 hour visa free transit and you do not need any kind of visa.

If you are Australian citizen and have an itinerary of this kind, you can stay in Nanjing and even longer than 24 hours in entire Shanghai area (Nanjing belongs to Shanghai transit area) or to be precise if your entire transit time in China is longer than 24 hours, you would have been eligible for 144 hour visa free transit.

So, if your layover is shorter than 24 hours, counting from your first scheduled arrival in China, until your last scheduled departure from China is shorter than 24 hours, you will be covered by 24 hour visa free transit, but if it is longer, you can use 144 hour visa free transit.

You will maybe want to exit the airport during your first layover and you are informed that you can do that by applying, immediately upon your landing, for a temporary entry permit at the transit counter to be allowed to exit the airport and to make a stroll in the city. This application is free of charge.