He had been drinking immediately before the trip, and was wearing jeans at the time.
He was described as an "exuberant character" who was happy to be in New Zealand and out on the trip.
"Mr Trendall appears to have been extremely exuberant and happy to find himself where he was to such an extent that he threw some Australian dollars out of the boat, joking that he would not need them anymore because he was not going to go back."
After being told to pull his head in by his two companions, Ian Richardson and Bill Jenner, Mr Trendall sat down before deciding to go for a swim.
He emptied his pockets and jumped into the lake about 90m away from the Maori Creek area, waving and showing his companions he was content.
Mr Richardson and Mr Jenner boated across to Maori Creek for a cigarette while he was swimming.
After a few minutes, the pair signalled to the swimmer it was time to go, who began making his way towards the shore.
"Mr Trendall was swimming towards them, he stopped, and he simply disappeared. There was no splashing and no sign of struggle, and it does not appear that Mr Trendall even looked up at them, that he was simply swimming, and stopped, and sank."
After the two men raised an emergency alarm, police, divers, helicopters and search crews combed the lake in an "exhaustive" search but could not find the body.
The coroner said the death needed to be considered as accidental.
The boat had appropriate safety gear and Mr Trendall, a strong swimmer, acted in a way that he intended to return to the boat after his dip.
She said police paperwork had suggested a number of explanations, from Mr Trendall suffering from swimming cramps and the possibility of a medical event, but said it was pure speculation.
"We cannot know why he disappeared and drowned as he did."
