The best time to go on a South Africa safari is during the May to October winter months. The vegetation is at its most trampled and thinnest at this time so game viewing is far easier than during the lush, green summer months. Day time temperatures are mild to warm (maximum 20 to 23°C) while nights are comfortably cool. Note that the later dry season months of September and October often experience extremely hot and dry conditions.
There's also very little winter rainfall in South Africa's northern safari destinations (such as the Kruger National Park and KwaZulu-Natal's reserves) which greatly reduces the risk of malaria. Although these destinations do lie within the malarial belt, the chances of contracting the disease while on safari, even in mid-summer, are extremely slim indeed.
If you want to include some time on the beach during your South Africa safari, it's important to remember that the beaches of Cape Town and the Western Cape are at their warmest, driest and sunniest from November to mid-March. Luckily, the warm sub-tropical waters of KwaZulu Natal's coast and the Eastern Cape make for a great beach holiday at any time of year.