"We later discovered that he had left his yacht on autopilot while he was seeing to something below deck. Who would leave their boat on autopilot in a harbour packed with boats fishing?" Nik Mimilo said.
Mr Mimilo's son, James, his cousin and Nik Mimilo jumped in the sea to keep Mr Mimilo afloat.
They pulled him from the water unconscious but breathing.
"There was a yacht coming straight towards us. No one seemed to be at the helm," said James Mimilo.
"We were screaming at him but he was heading straight towards us. At the the last minute he saw us but my dad already made the decision to jump. He could have killed my dad, my aunt and uncle and me."
Last night, James Mimilo said his father was in a stable condition in Auckland Hospital. He had lacerations on one leg, but no broken bones.
The accident occurred near Crusoe Rock, a popular fishing spot in Sergeant Channel, midway between Motuihe and Waiheke Islands. Conditions were clear and winds of 10 to 15 knots.
The Coastguard was alerted at 2.40pm by a member of the public and sent a rescue vessel to collect Mr Mimilo.
A paramedic and doctor from the Westpac rescue service were also rushed by the police launch Deodar to the coastguard vessel.
Mr Mimilo and his son were transported back to Mechanics Bay where the injured father was taken by ambulance to Auckland Hospital.
Coastguard operations manager Ray Burge said the accident was a reminder to all boaties to maintain a good look out at all times and indicate intentions early on.
Police spokeswoman Noreen Hegarty said the incident was being investigated by the Police Maritime Unit.
"Police will speak to those involved as well as witnesses to the incident but it could be several days before any decision is made around whether anyone will be held to account for their actions," she said.
