The cases were of sisters who had in January travelled to Malawi, where a cholera outbreak since last year has claimed more than 1,000 lives as of January, the highest on record in the country.
"Both patients had developed symptoms on their return to Johannesburg," the health department said in a statement.
"A close contact (household family member) of one of the patients was admitted to hospital on 4 February with diarrhoea and dehydration, and is considered a possible case," it said, adding laboratory test results were pending.
Cholera is an acute diarrhoeal infection caused by the bacteria Vibrio cholerae and can be deadly if left untreated. It is mainly spread by contaminated food and water.
Cholera is not endemic in South Africa, the health department said. The last outbreak in the country was in 2008/2009 when about 12,000 cases were reported following an outbreak in neighbouring Zimbabwe which led to a surge of imported cases and subsequent local transmission.
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