The son was rescued by members of the public and was in hospital on Sunday afternoon.
"He is expected to make a full recovery," a police spokeswoman said.
The man was taken to Wellington Hospital in a critical condition and died a short time later.
His death will be referred to the Coroner.
Police said they still had to notify members of the family before publicly naming the victim.
READ MORE:
• Between the Flags: Auckland woman, 86, confirmed as sixth holiday drowning victim
• Our grim toll: Drowning deaths reach four in three days
• Six people drowned over the holidays - two more than last year
• Far North drowning tragedy: Coastguard volunteer's desperate bid to save woman's life after boat flipped
• Between the Flags: Too many Kiwi blokes make bad decisions in the water
Residents around Marine Parade described several ambulances, police cars and a fire truck blocking off the street as ambulance staff performed CPR for a lengthy period of time on Saturday night.
A small crowd of onlookers had gathered on the wharf during the incident.
One neighbour, who asked not to be named, said a child could be seen wrapped in a towel at the scene.
Mark Cleverley, who lives a few doors down from the wharf, said he first noticed a gathering of emergency services as the sun set.
Once two ambulances arrived he knew something terrible had happened, he said.
But swimming around the Seatoun Wharf was generally considered safe by locals, Cleverley said.
"What happens is the youngsters jump off the wharf all the time. There might have been a bit of jumping there and someone's hurt themselves," he said.
"It was pretty full-on down there with all the cops."
Cleverley said there had been some of the usual Wellington winds blowing.
"It's pretty open there."
Several other residents described the sea as typically choppy and wild, and said the beach had been quiet throughout the day.
"I'd be surprised if anyone had gone swimming out there," one said.
Senior lifeguard Jonathon Webber, who is an advisory board member for Drowning Prevention Auckland, said the incident was "a real tragedy" and highlights the phenomenon of "aquatic victim instead of rescuer syndrome".
He said such scenarios, where someone dies trying to help another person, result in about 2 per cent of fatal drownings.
"Typically the person in trouble survives and the would-be rescuer fatally drowns."
Those who notice someone in trouble in the water, Drowning Prevention Auckland advises, should first check for danger before attempting to provide the person with flotation and calling 111.
Rescuing from land or a craft is safest. If entering the water try to take something that floats with you.