Nielsen won the gold medal at the 1997 FIBA Under-21 World Championship with Australia's junior national team. He was also a member of the senior men's Australian national basketball team. With Australia's senior national team, he won gold medals at the 2001 Goodwill Games, the 2003 FIBA Oceanian Championship, and the 2005 FIBA Oceania Championship. He represented Australia at the 2004, 2008 and 2012 Olympics.