Asking about Transit Phnom Penh- Shanghai-Auckland

If Shanghai is your only stop within mainland China, you are definitely covered by 24 hour visa free transit and you do not need any kind of visa. Since your layover is a long one and if you want to exit the airport for an overnight or just for a stroll in the city, you will approach 24/144 hour visa free transit counter immediately upon your landing in Shanghai where you will apply for a temporary entry permit to be allowed to exit the airport. This application is free of charge. The issuing of that permit is based entirely on discretionary decision of Shanghai airport authorities, so you are advised to support your application with a hotel booking confirmation. If you do that, you should have that confirmation in a printout form. In any case, the most important detail is that you do not need any kind of visa.