ajohn

Yes, this is macronyx.

Note that the usual name for this subspecies is Manchurian Yellow Wagtail. Grey-headed usually refers to thunbergi - this has not yet been recorded in Hong Kong but is possible here.  It is almost impossible to separate from macronyx though (thunbergi is supposedly slightly less bright yellow, with narrower whiter wing bars, a very slightly different head pattern and a slightly shorter hind claw)