Single Entry Visa Granted for 12 month stay

Hello I have been issued a professional Z type, for a job working in HK, the entry permit label I have received is for a single entry with twelve months. I want to travel home in a couple months and visit some parts and possible visit friends in OZ over Christmas. On this entry permit it appears I can't leave HK. It that true. Is it possible to apply for additional entry and exits or will I now be stuck. Be great fun of any advice that can be given,