The 52-year-old man was arrested last December, after 1.7kg of methamphetamine was found in his backpack at Denpasar International Airport in Bali. He had flown to Hong Kong to meet his internet girlfriend "Jessy".
"The things that I've heard people say online and to my face have astounded me," Mr Watt said.
People would say: "He should have known what he was doing."
"Well, he didn't, that's the point," Mr Watt said.
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."
Mr Watt said the prosecution "weaved tales" and had no evidence of any intent. Throughout the 450 pages of chat logs with Jessy, 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.
"If it was a court in New Zealand they would've walked out that day because there would be no case to answer."
The court 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.
"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."
De Malmanche had been speaking to Jessy online for about four months last year but was blinded by love and didn't see the signs, Mr Watt said.
"It was hook, line and sinker."
A lot of people were working on the de Malmanche case, for free or little reward, Mr Watt said.
