An investigation has been launched after the discovery of a man's body at the downtown ferry terminal in central Auckland.

Police were called at 8.02am when a member of the public spotted the dead man under Pier 3 in the terminal.

The body has since been removed from the scene for a post-mortem examination, but a heavy police presence remains in the area.

A police spokeswoman said the results of that post-mortem examination would reveal more details about how the man died, where he went into the water and how long he had been there.

Officers were staying at the scene as it was possible a forensic examination would be needed, she said.

She did not know what condition the body was in when it was found.

Ferries from West Harbour and Pine Harbour were diverted after the body was found, the Herald understands.

A Fullers spokeswoman said all ferry services were now running as normal.