Visiting Hong Kong on South African passport & going to Shenzen for a day, do I need a Chinese Visa?

We live in Manila (Philippines) for a couple of months and are going on a long weekend trip to Hong Kong and I know that we have 30 days entry without a visa with our South African passports. We are going to do a daytrip to Shenzen, do we need a Chinese visa and what is the easiest / quickest way to obtain one?