Makhubela said the brother became suspicious when his sister went to the neighbour's house several times.

A 34-year-old man was arrested on Friday morning for allegedly raping a mentally disabled teenage girl in Zondi, Soweto, police said.

"We had an incident where a mentally retarded girl, aged 17, was allegedly raped by a neighbour," spokesman Warrant Officer Kay Makhubela said.

"It is alleged that the neighbour was doing that every now and then when the girl came from school and he called her over to his place. He was arrested this morning after the girl's brother became suspicious."

Makhubela said the brother became suspicious when his sister went to the neighbour's house several times.

He said the brother suspected that the man might be raping his sister.

"We are not sure when it stated but they say it happened several times. The brother went to tell the parents but they did not believe him. They asked the girl and she told them what happened," he said.

"The girl was taken to a police station and a medical doctor confirmed that she was sexually assaulted."

Makhubela said the girl was taken to a place of safety and would receive continuous counselling.

The man is expected to appear in Protea Magistrate's Court soon.