A shoreline search was due to start at daylight and police said at 8.30am the search was ongoing.
"It is possible that the person who fell may have made his way to safety unaided, and may be unaware of the concerns for his safety. If this is the case, police ask that anyone who can provide any information contact them urgently," police said in a statement.
Police boat Deodar, land patrols and the Westpac rescue helicopter conducted a land, sea and air search from the bridge to Paremoremo, in the northwestern corner of the harbour.
Police were also looking for two other people seen with the person who fell just before the incident. Police search and rescue teams will undertake a shoreline search this morning in daylight.