The pair headed under Maungatapu Bridge up the channel to fish in Rangataua Bay.
The boat had a buoyancy rate of 180 kilos, whereas the men's combined weights were 262 kilos, the fishing weights totalled 110 kilos, and the boat was also carrying three heavy fishing nets.
When they left the shore the boat was mere centimetres above the waterline.
While feeding out the second net a small wave came over the stern but there was no bailing equipment, no lifejackets, radio or a cellphone on board.
Moments later the boat sank and both men ended up in the sea about 100m from shore.
The deceased told his son to swim to shore and he would follow him, but he never made it.
Mr Vahai junior estimated he was in the water for about 35 minutes. Police were notified at 7.56pm and despite an extensive search his father's body was not found until the next day on sandbank 200m off shore from Matapihi Bridge.
Dr Bain has reserved his findings, but noted Mr Vahai suffered from chronic ischemic heart disease which he said may well have contributed to his death.
On Tuesday he also heard evidence about the death of Shaun Hogarth, 23, who lost his life while fishing with three mates in rough seas in a stolen small dinghy on October 16, 2012.
The group had been drinking, there were no lifejackets or oars on board, and when the boat began sinking they tried to use empty beer bottles and an empty tackle box to try and bail out the water.
Mr Hogarth was last seen swimming back to the submerged dinghy by one of his associates.
His body was recovered by the Coastguard nine days later.
Dr Bain, who reserved his findings, said it was clearly a "tragedy waiting to happen".
He also reserved his decision in the inquest case of 56-year-old charter yacht skipper Richard Rusbatch, also presumed drowned after he went missing at sea last February without trace.
Mr Rusbatch's leased 38-foot charter yacht Honfleur was found motoring in circles 11 nautical miles east of Mayor Island at 8.50pm on February 14, 2012.
Dr Bain said given the evidence he has been received, there does not seem to be any doubt that Mr Rusbatch had fallen off the boat, and was dead.