Transit at Hong Kong Airport

I am arriving into Hong Kong on 14th Oct morning at 1105 on my way to Sydney. The CX flight is connecting me at 2130 hrs. Since there is a gap of 10 hrs for my flt to Sydney, would it be possible for me to come out of the airport and do some shopping at the ladies market in Mong Kok for about 5 to 6 hrs? If so what is the quickest transport that i can take and come back into the airport by 1830 hrs. I was told that the Airport bus A21 would take me. Can somebody help?