UPDATED 2.32PM: The body of a man has been found in Wellington Harbour this morning after he jumped from a 30 metre-high crane.

The 20-year-old man is believed to have jumped from the floating crane, the Hikitia, at around 7.

Police say many people saw him enter the water but he did not resurface.

The Crane Association says the crane has become a target of young thrillseekers, and today's incident is the result of what can happen when things go wrong.

A woman, who does not wish to be named, said a crowd of around 50 people watched as emergency services searched for the man.

She said a friend had dived into the water to rescue the jumper after he did not surface from the water. A next of kin is being identified.

The incident follows a video that appeared on Facebook last week, of another person jumping from the crane.

The jumper, who has not yet been identified, survived the plunge, but police condemned their behaviour.

"The most concerning thing is the safety issues. Climbing on to that boat with all the associated equipment on the deck etc, and then clambering up the crane to the peak. The potential is there to fall down on to the crane," Inspector Dave Rose said.