The photos are tricky, presumably the birds were distant.
The exact details of the plumage are hard to judge, but from the wing pattern, they look like Garganey to me. In particular, some birds have pale greyish forewing and broad white bands on each side of the speculum (typical of male Garganey).
Garganey would also be likely based on occurrence - it is the commonest duck in Hong Kong during September, and there have been large numbers recently after the passage of Typhoon Mangkhut (I saw almost 350 at Mai Po this week, but haven't seen any Wigeon yet this autumn).
