The South African Police Service (SAPS) said in a statement that the shooting took place in Kwazakele, Gqeberha, in the Eastern Cape province on Sunday, with two gunmen opening fire at the assembled guests.
Police said that the shooting took place between 5.15pm and 5.30pm local time and the gunmen fled after the attack.
Police said that no arrests have been made yet and the incident was being investigated.
While the victims have not been identified yet, authorities said that the owner of the house was among them.
Eastern Cape SAPS commissioner Nomthetheleli Lillian Mene said: “These victims were killed by criminals, and we will not rest until we find out what happened and who was responsible for (this) callous and cold-blooded attack on these unsuspecting victims.”
The incident is the latest in a spate of mass shootings last year.
Last July, gunmen killed 19 people in random shootings within hours of each other.
According to Gun Free South Africa, there are about 3 million guns registered in the country.
However, the group said that the number of unregistered guns are far more.
South Africa has one of the world’s highest murder rates, with around 20,000 people killed every year out of a population of 60 million.
(Additional reporting by agencies