A man has been left with serious head injuries after he was assaulted with a hammer in central Auckland yesterday morning.

He was sitting on a bench near the corner of Lorne Street and Rutland Street outside the Auckland Central Library at the time of the attack.

Photo: Police

Police said an unknown man came from behind with a hammer and assaulted him about 8.50am yesterday.

He then fled the scene towards Mayoral Drive.

The man was taken to hospital and is now in a stable condition.

Acting Detective Inspector Glenn Baldwin said it was a shocking, unprovoked and cowardly attack.

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

Police have also released a CCTV image of an individual they believe can help with their inquiries.

Mr Baldwin said anyone with information should contact Auckland police on 09 302 6557.

Or call Crimestoppers anonymously on 0800 555 111.