MSF says unaccompanied children are increasingly crossing the border
Zimbabwean refugees in South Africa are being denied access to much-needed medical care, says the aid agency Medecins Sans Frontieres.
MSF says it has many reports of refugees who have been raped and injured being rejected by hospital staff or charged exorbitant fees.
The group describes the situation as a failure of the South African government and the UN to protect the Zimbabweans.
MSF said lone children are increasingly crossing the border and being abused.
The inauguration in February of a fractious power-sharing government in Zimbabwe has not stemmed the flight from what was once one of Africa's most prosperous nations.
Some three million Zimbabweans - about a quarter of the entire population - are believed to have crossed the border to escape the economic collapse and human rights abuses at home, as well as a cholera outbreak that has infected about 100,000 people.
South African authorities announced in April that Zimbabweans could get permits to stay legally in South Africa for six months, entitling them to work, healthcare and education.
But MSF said many Zimbabwean migrants in South Africa were still being "treated very poorly".
The agency said it had medical teams working at the border town of Musina and at Johannesburg's Central Methodist Church, a "de facto refugee camp" in the heart of the central business district where thousands of Zimbabweans seek shelter every night.
"Our medical teams see a shocking array of illnesses and they hear stories from our patients which are quite horrifying," the MSF head of mission in South Africa, Rachel Cohen, told the BBC's Network Africa programme.
She said that as well as high rates of sexual violence, HIV, tuberculosis and cholera, there was a growing problem of unaccompanied children crossing the border who were "exposed to many forms of abuse and violence".
The MSF mission chief said the influx of Zimbabweans was placing a huge burden on South Africa's health system, but that refugees should nevertheless be entitled to access healthcare.
"In fact, what happens though is that they are often rejected, they're charged exorbitant fees, they're discharged prematurely, they're treated very poorly by the staff," she said.
"And what we're trying to say today is that we are witnessing daily a kind of failure of the South African government, and also United Nations agencies that do have a mandate to protect refugees, asylum seekers and so forth."