It is illegal for a person on a dependant's visa (of a person on a work permit) after July 1, 2003 to take up any type of employment even if it is volunteer work without the permission of the immigration department. Additionally, it is illegal for the person on a work permit to take up any additional paid or unpaid work. Any employer is legally required to check the passport and type of visa of anyone they hire. If they don't they are liable for a $150,000 fine and one year in prision.
Typically if your caught you'll go to prision for 6-8 weeks and then get deported. The person hiring you is also liable and could go to prision and pay a fine (max. 3 years and $350,000).
A word of advice, if your wife is qualified she could get hired by a tutorial centre and they could get her a work permit.
You can get more information by going to www.immd.gov.hk and then click on the link to the immigration department.