Neil Prakash, who was detained one year ago inside Turkey close to the Syrian border, is jailed in the Gaziantep region of southern Turkey.
However, his trial, which got under way earlier this year, is being held in the neighbouring Kilis region with Prakash being cross-examined via video link.
In his testimony, Prakash said he said received training from IS in their de-facto capital of Raqqa in Syria before moving north to the town of Kobane to fight Kurdish militia.
He was wounded in the fighting and then requested to be moved to another area where he did not have to fight.
"But I went to Raqqa and was told I had to fight," he said, quoted by the Turkish-language Dogan news agency. "I was also made to speak in propaganda videos.
"I decided to escape after seeing the true face" of IS, he added. "I very much regret joining the organisation," he said.
Prakash denied being the Australian "representative" of IS in Syria and asked to be released and deported to a Muslim country and not Australia. However the court decided to keep him in custody, Dogan news agency said.
The trial was adjourned to an unspecified later date.
Australia last year had asked Turkey to extradite Prakash, who Canberra previously had reported as having been killed in a US airstrike in northern Iraq.
Prime Minister Malcolm Turnbull has called Prakash the senior Australian operative in IS. He was reportedly on a list of high-value IS recruiters targeted by the US in drone attacks in Iraq.
Prakash told the court he had spent his early life in Cambodia in a Buddhist family but had decided to become a Muslim after watching IS videos.
Turkey had been under fire from its allies for not taking a hard enough line against jihadists on its territory but stepped up arrests from 2015 after a string of terror attacks.
