Forty-two Zimbabwean and Malawian nationals have died after a bus taking them home overturned on a South African road, authorities have said.

The crash happened on Sunday night as the bus was moving through "a mountainous section" of the N1 highway in South Africa's Limpopo province, local transport officials said.

"It [then] veered off the road along a steep mountain pass and plunged down an embankment," they added.

South African President Cyril Ramaphosa mourned the high death toll, which includes seven children, and said this was not only a tragedy for the country but for "our sister states of Zimbabwe and Malawi" as well.

The cause of the accident is unknown, but an investigation has been launched.

According to reports, the bus was travelling from the city of Gqeberha in the Eastern Cape.

It is not clear how many people were travelling on the bus when it crashed, or its capacity.

As well as the seven children, 18 women and 17 men died, while 49 people were injured, authorities confirmed.

Government officials, including Transport Minister Barbara Creecy, are set to visit the scene of the crash as well as the injured passengers.