Tai Samuel Neilson, 25, and Adam David Kearns, 23, were charged after Marshall was attacked on April 18, in Queenstown.
Neilson, who appeared in Christchurch District Court on Monday, has pleaded guilty to the assault.
He will appear in the court again for sentencing on August 20.
Kearns has pleaded not guilty to the attack and is due to appear in court again on July 14.
Marshall, 40, played 81 Tests for the All Blacks between 1995 and 2005. He now works for Sky Sport as a rugby commentator.