Police have released CCTV images following a "shocking, unprovoked and cowardly" hammer attack on a man sitting on a bench in Auckland.

The assault took place around 8.50am yesterday when a man approached another from behind, striking him with a hammer near the intersection of Lorne St and Rutland St.

Acting Detective Inspector Glenn Baldwin said the attack was an alarming incident.

"This was a shocking, unprovoked and cowardly attack on a victim who was unaware and attacked from behind," he said.

"We want to find the dangerous offender who was responsible for this vicious attack and hold them to account.

"We believe the person in this image can help our investigation and we need your help to identify them."

The attacker snuck up on the man, coming up from behind him and assaulting the victim with a hammer before fleeing the scene outside Auckland Central Library.

Police report the man ran away up Rutland St towards Mayoral Drive and the victim has serious head injuries and is in a stable condition in hospital.

Anyone with information or any members of the public who witnessed yesterday's incident are asked to contact Auckland City Police on 09 302 6557.

You can also contact Crimestoppers anonymously on 0800 555 111.