[1] He won a gold medal in the 100 m breaststroke at the 1999 All-Africa Games, and later became a top 8 finalist in the same distance at the 2000 Summer Olympics.

While studying in the United States, Petersen was part of the 200-yard medley relay team that claimed a top finish at the 1998 Atlantic Coast Conference Swimming Championships.

[3] On that same year, Petersen powered home with a gold medal for South Africa in the 100 m breaststroke (1:02.63) at the All-Africa Games in Johannesburg.

Swimming a breaststroke leg in heat two, Petersen recorded a split of 1:02.51, but the South Africans finished the race in fourth place and thirteenth overall with a final time of 3:42.44.

[11] At the 2001 Goodwill Games in Brisbane, Petersen fought off a challenge from Australia's Simon Cowley to pick up a silver medal in the 50 m breaststroke (28.72).