After the 1999 failure of New Zealand's campaign in the World Cup, in which John Hart played him in an unfavoured position, Christian sustained a severe injury to his knee. Cullen received an operation on his knee and underwent serious recovery therapy. After being out the better part of a season, Cullen returned to the field. Although he still played with flair and passion, many argued that the operation had taken its toll and Cullen was no longer at his peak. Cullen however still continued to impress and played at an international level up till 2003. At the conclusion of 2003 he moved to Ireland where he played for Munster.
His biography, Christian Cullen: Life on the Run (by John Matheson) was released in October 2003. The book sparked significant media comment for Cullen's criticism of All Black coach John Mitchell.