Chinese Bush Warbler is the name usually given to a rather different species Bradypterus tacsanowskius, which is basically a West China bird.
This photo is of what is sometimes called Japanese Bush Warbler, or sometimes Manchurian Bush Warbler Cettia diphone.
For more details on ID see Viney, Phillipps and Lam Birds of HK and S China, and on status see Carey et al Avifauna of HK
Mike Turnbull