In fact it is Eastern Great Egret which occurs in Hong Kong. Western occurs as far east as NE China and Japan (where Eastern also occurs!), but not into South China. The official split between these species is relatively recent, but is now accepted in Hong Kong. Western may be a potential vagrant, so it is worth trying to get familiar with how to separate the two.
Trying to separate these two is likely to be difficult. I agree that the bird photographed looks to have a very long neck and bill, but have not seen this as an ID feature. According to Brazil's 'Birds of East Asia', Western is larger with yellow on the legs in the non-breeding season. That MAY suggest this bird is Eastern, but I am not sure.
