Here is the place to find information regarding the ban.
Here is the answer to your question.
If I have a large-capacity magazine, do I need to get rid of it?
No. Continued possession of large-capacity magazines (able to accept more than 10 rounds) that you owned in California before January 1, 2000, is not prohibited. However as of January 1, 2000, it is illegal to buy, manufacture, import, keep for sale, expose for sale, give or lend any large-capacity magazine in California except by law enforcement agencies, California peace officers, or licensed dealers.