P.R.C Passport travelling to Hong Kong as a transit ticket

Hi, I am travelling to from Melbourne to Hong Kong (as a transit point) then flying to Wuhan, I want to stay in Hong Kong for 15 to 20 days, please advise what forms do I need to fill in and is there a limit in the days I could stay in Hong Kong? Also what are the costs involved? I will have a sponsor in Hong Kong. Thanks! Daisy.