Running the risk that your very own work visa won't be extended, among others.
Just don't hire any one who has overstayed. ALWAYS ask to see their visa. Getting caught hiring an overstayer is big trouble for you as the employer and the girl. Immigration really put the boot in if they get a girl on two counts - working in breach and being over. You as the employer also face prosecution.
As long as the girl has not overstayed, even if she gets stopped in the street by Police or Immigration as long as she produces a valid ID the boys in blue tend to take no further action. They can't prove anything provided she is not working in a shop or restaurant. Heard about a few cases like that where the employer of the helper was using the girl as cheap labor in shops or restaurants. Automatic custodial sentences for both. Never heard about any girl or employer being prosecuted for part time work though.
If the OP lives alone I would recommend getting a part time helper. There are many helpers only too willing to earn extra money to supplement their meagre income.
Really don't see the point in hiring illegally.
There are plenty of part time helpers holding permanet HKID - just hire one of them.
i employ two local ladies to clean my playgroup centre everyday. wasn't that difficult to find them.