Cyclone Kenneth has made landfall in northern Mozambique on Thursday after sweeping through island Comoros.
Torrential rain of up to one metre is expected over the next five days with winds of up to 225km/h, according to Gauteng weather.
The category four cyclone is reportedly the strongest cyclone to hit Mozambique with flooding and landslides expected in the devastation.
Compulsory evacuations have already been effected.
Comoros experienced power outages, and homes and roads have been destroyed.
Relief aid organisations are on standby and some, like the Red Crescent, have already begun with support in some of the affected areas of Comoros.