It's an Eastern Marsh Harrier, I believe, an adult female.
They have a wide range of plumages, and this one does not in this photo show the warmer, redder tones which this species often/usually has in this plumage type, based on experience and corroborated by reference to Brazil's Birds of East Asia (an essential reference in our region, if not quite the final word on bird id hereabouts - and actually we are just outside his range of coverage, which includes Fujian, but not Guangdong).
It looks too heavy for Pied, and perhaps for Hen (which may well occur in HK as a very scarce visitor, but is not currently on the list), though it seems to me to be difficult to completely exclude those on this photo.
I'm sure some with more recent, regular experience of these species, may have further comments.
Mike Turnbull
