More than 440 people have died in floods and mudslides in KwaZulu-Natal province, where thousands of South African soldiers are missing.
After the torrential rains began, the South African National Defence Force said it had been instructed to activate 10,000 troops for tasks including mop-up work and transporting aid.
The military will also provide medical support and helicopters for rescue and reconnaissance missions.
The floods left thousands homeless, knocked out power and water services, and disrupted operations at one of Africa's busiest ports, Durban.
President Cyril Ramaphosa said in a televised speech that his cabinet had declared a national state of disaster because of the floods and extreme weather in several provinces, which would allow for a more effective response to the crisis.
He said the Finance Minister would approach parliament about additional spending, over the 1 billion rand $93 million he had said was immediately available.
Herdsman Mbukeni Khwela accompanied police officers and sniffer dogs to scour a river for a missing neighbour who had been swept away from the town of Umbumbulu about 45 kilometres south-west of Durban.
She has found her son, but she hasn't found her, the 59-year-old said.
The latest death toll from the floods was at 443, with 48 people still unaccounted for, according to Ramaphosa.
The officers and a sniffer dog who had been killed in the floods were among the worst to affect the east coast province in recorded history, according to the police.