Joyce Kgoali, chairperson of the National Council of Provinces, died on Sunday morning following complications after a minor operation, President Thabo Mbeki's office announced.
Spokesperson Bheki Khumalo said her death was "completely unexpected and shocking".
Kgoali, who was born in Lesotho and 54-years-old, had taken over the post from Education Minister Naledi Pandor.
The African National Congress (ANC) said in a statement that she was also a member of the ANC Women's League National Executive Committee (NEC) and the ANC Gauteng Provincial Executive Committee.
"She started her revolutionary life as a trade unionist in the textile sector. She later joined the United Democratic Front (UDF) in what is now Gauteng. She was active in the Federation of Transvaal Women (Fedtraw), and later became a member of the ANC Women's League."
She was also an underground cadre of the ANC involved in a number of activities.
"Following the 1994 election, Kgoali became a member of the Senate (later the NCOP), representing Gauteng. She became MEC for Public Works and Transport in Gauteng in 1998. After the 2004 election, she became chairperson of the NCOP."
Kgoali had been deployed in KwaZulu-Natal when the ANC in the provinces was preparing for its conference.
"Those with her would remember her as a person prepared to work throughout the night to ensure the success of the conference. At the time of her death, she was deployed in the Free State province assisting the ANC structures with the many challenges they face," the ANC statement read.
"The national caucus of the ANC will remember her work in parliament, her dedication and her firm voice on important issues." - Sapa