Mr. Ullah was later charged with six terrorism offenses, including training people in the use of encrypted apps, and was sentenced to eight years in prison after admitting to being a member of the Islamic State.

Damien Spleeters, the head of regional operations at the investigative group Conflict Armament Research, who has traveled to Iraq and Syria to investigate the provenance of weapons used by the Islamic State, testified at Mr. Zahab’s trial that he found fuel propellants and rockets at ISIS workshops that mirrored what he later learned were Mr. Zahab’s designs. The terror group, he said, was manufacturing its own rockets and its own fuel.

“What we saw is that some of the pictures I took of some of the rockets I found in Iraq are actually quite close to some of the designs,” Mr. Spleeters said in an interview. “The research he has done was pretty close to what ISIS used in the field, it was the same precursors of chemicals used to make explosives.”

Australian Federal Police officers interviewed Mr. Spleeters about his research in London in June of last year. That was more than a year after Mr. Zahab’s arrest. Justice Bellew said in his ruling on Friday that the defendant’s use of encryption prolonged the investigation by nearly a year.

When investigators searched Mr. Zahab’s farm in the town of Young in February 2017, they found bags of potassium nitrate, which can be used to make propellant, and containers of ammonium persulphate, which can be used to make electronic circuitry.

Justice Bellew said he was not convinced by Mr. Zahab’s statement that he had renounced the Islamic State. Mr. Zahab said that he had been caught up in an internet bubble and had seen the terrorist group as a force for good.

Asked on Friday whether he believed Mr. Zahab had genuinely de-radicalized, his brother, Tarek Zahab said: “I don’t think he was radicalized at all.”