Koertzen was born in Knysna, Cape Province, Union of South Africa, on 26 March 1949.

Both matches were contested between South Africa and India at Port Elizabeth, in the first series in which television replays were used to assist with run out decisions.

[5][6] This nickname originated from Daryll Cullinan, the South Africa batsman, who once asked Koertzen: “Why do you make me suffer and wait for that slow death decision?”[7] Koertzen became a full-time International Cricket Council (ICC) umpire in 1997, and was one of the original members of the Elite Panel of ICC Umpires when it was founded in 2002.

[16] Koertzen's professionalism saw him through some controversial moments: in September 1999, he refused a bribe to fix the outcome of the final of the Singapore Challenge between the West Indies and India, and in January 2000, he stood in the Test match between South Africa and England at Centurion, where both teams forfeited an innings in order to force a result after South African captain Hansie Cronje had been approached by a bookmaker.

[20] On 4 June 2010, Koertzen announced that he would be retiring from umpiring after the Test series between Australia and Pakistan in England that summer.

His last representative match as an umpire came during the 2011 Indian Premier League between Royal Challengers Bangalore and Chennai Super Kings.