The Bamboos agency has good Cantonese-speaking local nannies. They are mostly middle-aged women who are experienced "pu yuet" or part-time nursing assistants at hospital maternity wards.
But, as Carang points out, they are much more expensive than a helper.
Bamboos' nannies cost from HK 600-900 a day, and they don't do housework.
We use them on occasion if I have to work on a Sunday or public holiday.
But for the main childcare, we have a full-time domestic helper.