Colthorpe went along with the U-19s national side to Thailand to participate in 2004 FIFA U-19s Women's World Championships. Colthorpe then made her first full debut for the Matilda's in November 2005 in Gosford, New South Wales when Australia defeated China 3–1 in a friendly match. Colthorpe was a part of The Matilda's side that reached the quarter-finals of the 2007 women's world cup. In the quarter-final against Brazil, she scored with a header to level the score at 2–2 before Brazil eventually triumphed 3–2. In 2010, Colthorpe helped Australia win their first silverware in the Asian Cup, mainly playing out of right fullback for most of the campaign.