Africa's most industrialised nation is a magnet for undocumented migrants from across the continent, some of whom are lured into schemes in which they are held against their will.

The four Chinese men and three women were arrested in November 2019 after police raided a Johannesburg factory called Beautiful City which reportedly made cotton inners for duvets.

An investigation found that between 2017 and 2019, the factory employed 91 undocumented Malawians, including 16 aged between 14 and 16, prosecutors said.

They were forced to work 11-hour shifts, seven days a week, and confined to the premises under armed guard.

The seven Chinese were found guilty in February this year on 158 charges, including human trafficking and bondage.

In handing down the sentences, the judge David Mhango said the Malawians were "lured into the country, kept in bondage".

They were made to operate machinery which resulted in some of them losing their eyesight and limbs, he said.

South African police in May rescued 44 Ethiopians, 17 of them minors, being held against their will in an upscale neighbourhood of Johannesburg.

In March, dozens of young Ethiopian adults and children who were also allegedly held captive in a suburban house in Johannesburg escaped, with police finding 32.