Two homesteads in the town of Lusikisiki in the Eastern Cape were targeted, police said, with 12 women and one man killed in one location, and three women and one man at a second location.
An 18th victim is in a critical condition in hospital, the South Africa Police Service said.
The police minister, Senzo Mchunu, and commissioner are due to give an update at 13:00 (12:00 BST). Mchunu is also expected to visit the area where the attack occurred.
South African media outlets are reporting the victims were relatives and neighbours in Nyathi village, Ngobozana in Lusikisiki.
They said the group had been gathered at the houses to prepare to attend a traditional mourning ceremony for a mother and daughter who were murdered a year ago.
The victims had been packing goods and presents, including furniture, for the event when the attacked occurred on Friday night, according to the media reports.
News outlet Dispatch Live quoted local Ingquza Hil mayor Nonkosi Pepping saying: “The gunmen came and shot randomly killing everyone. Women and children were also killed in the bloody shooting.
“This has left the community terrified.”
Officials have also described the incident as gruesome and senseless, but have yet to determine the motive or make any arrests.
South Africa has one of the highest murder rates in the world, according to the latest figures from the United Nations Office on Drugs and Crime.
There were more than 27,000 murders in 2022 – amounting to 45 people per 100,000, out of a population of almost 60 million. By comparison, the US rate is six per 100,000.
