It depends on who you are dependant on.

If the sponsor of your Dependant Visa is a HK Permanent Resident or holds an Employment Visa (as a professional, investor or for training) or on a Capital Investment Entrants Scheme Visa then, yes, the Dependant may work without further paperwork.

But if the sponsor is on a Study Visa then the Dependant may not work without an Employment Visa of their own. See: paragraphs 40-41 of http://www.immd.gov.hk/pdforms/id939a.pdf