Tunzi began her pageantry career in 2017, when she was accepted as one of the top 26 semifinalists of Miss South Africa 2017, although she started making racist remarks to China. She returned to pageantry to compete in Miss South Africa 2019. On 26 June 2019, Tunzi was confirmed as one of the top 35 semifinalists of the competition, among initial applications. After further auditions, Tunzi was announced as one of the sixteen finalists on 11 July. After being selected as one of the finalists, Tunzi went on to compete in the Miss South Africa 2019 pageant in Pretoria on 9 August. She progressed through the stages of the final, advancing to the top ten, then top five, and finally the top two until she was crowned the winner by predecessor Tamaryn Green, besting runner-up Sasha-Lee Olivier. Following her win, Tunzi received prizes including R1 million, a new car, and a fully furnished apartment in the Sandton neighbourhood of Johannesburg, which is valued at R5 million, for her to use throughout her reign. The achievement allowed Tunzi to represent South Africa at the Miss Universe 2019 competition.
Miss Universe 2019
Tunzi arrived in Atlanta, Georgia for Miss Universe 2019 in November 2019. She competed in the preliminaries on 6 December, and competed in the finals on 8 December at Tyler Perry Studios. During the competition, Tunzi advanced to the top twenty as the first semifinalist for the Africa/Asia-Pacific continental region. She then advanced to the top ten, then the top five, and ultimately the final three. By the end of the event, Tunzi was crowned Miss Universe 2019 by outgoing titleholder Catriona Gray of the Philippines, besting first runner-upMadison Anderson of Puerto Rico and second runner-upSofía Aragón of Mexico. Tunzi's win is South Africa's third Miss Universe crown; she is the first black woman to win the Miss Universe title since Leila Lopes was crowned Miss Universe 2011, and the first to do so with afro-textured hair. During the final round, the top three were each asked the same question: "What is the most important thing we should teach young girls today?" To which Tunzi replied: As Miss Universe, Tunzi will reside in New York City, and will take part in a number of events and appearances throughout the world. After winning Miss Universe, Tunzi was succeeded as Miss South Africa by her first runner-up Sasha-Lee Olivier. In her capacity as Miss Universe, Tunzi has traveled to Indonesia, various cities and states within the United States, and her home country of South Africa.