Australian citizen plans to visit Korea for 3 months & ravel China during the time

Hello
I'm an Australian citizen residing in Australia. I have a plan to visit my family in Korea for max 90 days. During this time I would like to take a short trip to PRC with my niece who is a Korean citizen. I'm not required for the alien registration as Australians can stay in Korea for 90 days without a special visa.

May I know the procedure to get a relevant permit to travel PRC and max. days I could stay in PRC? Can any of the preparation be done while I'm in Australia? Thank you.