Do I need to check-in (luggage) again if I have a 12 hours layover? Paris-Pudong-Sydney

I hope you've booked through ticket. If so, the transit procedure can be much easier. As I know, passengers with Lufthansa, Air New Zealand, China Eastern Airline, SwissAir, and Virgin Atlantic don't need to claim their luggage at PVG. If not, they have to collect and reclaim the luggage.

As your stopover is 12h, you may wanna go into the city. Well then, the general transit procress is as follows:
Landing – International Arrival Access – Joint Inspection (get a stay permit which is needed if leaving the airport), Quarantine, and Immigration – (Luggage Claim) – International Arrival Hal - Leave PVG to the downtown – Security Check – Check in at boarding gate, which is clearly printed on your boarding pass – Waiting and Boarding
May help!