According to Beeld, Rossouw was admitted to hospital at the weekend with chest pains but was discharged a few hours later.
A friend found her body at her flat when she failed to arrive for a dinner date. The cause of her death was unknown.
M&G editor-in-chief Nic Dawes said: "Mandy was an extraordinary presence in our newsroom and in our pages; vivid, acute, and determined to break the story. Our thoughts are with her family, her Media24 colleagues and her many friends across the worlds of journalism, politics and diplomacy."
Rossouw was the international correspondent for the Media24 group, politics reporter for the City Press and was also a reporter for Eyewitness News.
She previously worked as a politics journalist for Beeld and the Mail & Guardian.
Last year, she released the book Kings and Kingmakers before the ANC's national elective conference in Mangaung in the Free State.
"Her death is a huge loss for us," said Tim du Plessis, head of Afrikaans newspapers at Media24.
Tweets flooded in on Tuesday morning from friends, colleagues and the public, expressing their shock at Rossouw's death. – Additional reporting by Sapa
Finding out a good friend& colleague has passed away, while on air, is hard. Cannot believe Mandy Rossouw is no more. Devastated. RIP Mands
— Redi Tlhabi (@RediTlhabi) March 12, 2013
So hard to believe that Mandy is gone. One of the most prolific and dynamic political journalists of her era. Rest in peace Mandibles
— Rapule Tabane (@RapuleT) March 12, 2013
Stunned and sad at the death of Mandy Rossouw.
— Nicholas Dawes (@NicDawes) March 12, 2013
Our dear, spirited and beloved colleague Mandy Rossouw passed away last night. May she rest in peace. Details when we know them.
— Ferial Haffajee (@ferialhaffajee) March 12, 2013