Do you need someone who speaks fluent or very good English? If so, it is unlikely that you will find someone in the local market. Best to speak with nanny agencies which can help you recruit someone, who will likely be from overseas, but sometimes they have nannies with permanent residency who can start immediately, which is likely what you will need. They can also source temporary legal childcare until your perminent nanny gets here. It took almost 8 weeks for our nanny to be granted a working visa for HK (coming from England with all proper qualifications for HK immigration and no follow up immigration queries that take more time). Expect to pay in excess of HK$15,000 at the low end for a proper nanny.
If you decide on a domestic helper, it will take a minimum of 4 weeks for her visa to be granted (assuming she is already in HK and at the end of a contract). If she is coming from overseas, expect more like 6-10 weeks for the visa to come through.