Mathew James Keliher, 18, pleaded has pleaded guilty in the Christchurch District Court to a charge of wounding with intent to cause grievous bodily harm.
He was with a group at the Colombo Street and Moorhouse Avenue underpass when he got into a verbal altercation with a 53-year-old man.
Keliher headbutted the man, pushed him to the ground and kicked him in the head before running away.
The victim was in hospital for a week with various broken bones, a fractured skull and a brain haemorrhage.
Keliher has been remanded in custody until his sentencing on May 31.