When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Johannesburg is roughly greater than that of Ste. Marie Marie. You can assume this to be the case over the next 19 days.
Expect the weather to be around 26 °C on average in January. As the temperature peaks, try to get your plans done early, as flight price and availability will have an impact on your travels. The climate in Ste. Marie Marie is generally colder than in Johannesburg.