Re travel out of China to HK & Return

Hi, I'm from Australia and have applied for Single entry visa as I will only be there for 27 days.
Now I think I'd like to go out of Mainland China to stay in HK for 2-3 days. Is there any way that once in Mainland China, I can get approval to go to HK and back, without re-applying for a new visa. I do not know the actual dates when I would be wishing to go to HK, so could not apply for a Double Entry Visa, as you must supply them with a full Itinerary of all your trip.

I can't really make HK my final stop then fly back to Australia, as my flights are direct, (no stops) into and out of Changsha, so after I am in HK, I must get back to Changsha to get my return flight to AU.