He then pointed the pistol to the right side of his head and dry-fired it twice. It was not known if the gun was loaded.
Mr Robertson later asked his son to go and get some more beer.
Mr Ferris sighed, but before he could answer his father said "Well if you won't f**king go, then by the time I count to three then I'll go myself''.
Mr Ferris replied saying he would get the beer but after he answered his father reached three and pulled the trigger shooting himself in the head.
Mr Robertson slumped forward in his chair and fell to the floor.
His son then called his partner and told her his father had just shot himself.
He called 111 and the operator suggested he carry out CPR on his father, who was breathing but bleeding from his mouth.
Mr Ferris decided against CPR as he knew his father, a former drug user and on the methadone programme, was a hepatitis C carrier.
In his ruling Mr Smith said Mr Robertson was a person who seemed to have "lost his way in life'' but was trying to reconnect with his son and grandchildren at the time of his death.
He said he and his son had enjoyed each other's company on the day of his death and there was no evidence he wanted to commit suicide.
He expressed his condolences to Mr Robertson's friends and family.
- NZPA