"The things that I've heard people say online and to my face have astounded me," Mr Watt said.
"You get 1000 comments of support and two or three against, but that's enough."
People would say; 'he should have known what he was doing'.
"Well, he didn't, that's the point," Mr Watt said.
"It is much better to err on the side of mercy and grace than it is to err on the side of judgment.
"Those people that are passing judgment on someone they don't know, they really need to step back and consider how they would react if it was their father, if it was their uncle, if it was their slightly intellectually disabled cousin who got deceived by this.
"They would be doing everything they could for them, whether or not they were guilty."
He has known de Malmanche for four years and been his minister for three.
Mr Watt testified in the court in early June, before he visited de Malmanche in Kerobokan Prison.
"My experience in the court was a joke. It was a farce," he said. "You definitely did not get the impression Tony was innocent until proven guilty."
Mr Watt said the prosecution "wove tales" and had no evidence of any intent. Throughout the 450 pages of chat logs with Jessy (de Malmanche's online girlfriend), which were presented by the defence, there was no mention of Bali.
"I'm just so absolutely frustrated at the feeling of having everything that we needed to prove that a man is innocent, but you're left running into a brick wall.
"They're just unmovable," he said of the Indonesian justice system he saw in Bali.
"If it was a court in New Zealand, they would've walked out that day because there would be no case to answer."
The trial sat for one day a week over about three months. On June 4, the defence had its one day to present its case, but in reality it was only about two hours.
Psychiatrists and New Zealand lawyer Craig Tuck were the first witnesses called.
"Then it was my turn to go in," Mr Watt said. "I had maybe a dozen letters, character witnesses for Tony ... the judge asked me to pick the best one and summarise it. That was my testimony."
He was then asked if he wanted to say anything to de Malmanche in the courtroom, where he was seeing him for the first time since December.
"I said 'we want to have you home, we love you and we're praying for you'."
Mr Watt was then asked some questions. "'How long have you known him? Was he happy?' That was the big focus," Mr Watt said. "In the time that I'd known him [in Wanganui], he was probably the happiest he'd ever been in a long time."
The day after giving evidence in court, Mr Watt visited de Malmanche in prison.
"He was in decent spirits. He's been there long enough to come to terms with the situation and find a way to live. That's really been the story of his life. He's just found a way to live."
De Malmanche had been speaking to Jessy online for about four months last year. He had mentioned it to his family and Mr Watt.
"It was a month before he left that he started talking about it. He'd kind of bring it up awkwardly," he said.
"The way I phrased it to him was, 'What's the catch?'
"(Now) he said he can't go to sleep some nights because he can hear me saying 'what's the catch?'"
But de Malmanche was blinded by love and didn't see the signs, Mr Watt said. "It was hook, line and sinker."
A lot of people are working on the de Malmanche case, working for free or little reward. Mr Watt said they were invaluable.
"We sat, at the end of the [day], around the table. It was such a disparate group of people. People from walks of life that would so rarely come together."
Mr Tuck has said the defence will be appealing but Mr Watt said that was not for him to comment on. "It's going to be a long 15 years," he said.
"The danger going forward is it becomes less and less important in people's minds. It's something to keep in the forefront of your mind but also not let it prevent us from doing other things that we need to do. And that's the advice I've given the family."