The retired major general was a driving force in the establishment of the Brereton war crimes inquiry, which found 25 Australians committed up to 39 murders in Afghanistan.
He told the royal commission on Thursday the issue of defence and veteran suicide was as "diverse and as complex" as people, and there was a time when he thought they were consequences of exposure to war and battle.
Defence and veteran suicide is as "diverse and as complex" as people, General Jeff Sengelman says. (HANDOUT/DEPARTMENT OF DEFENCE)
"That was the primary focus of my concern," he said.
"I now realise that it's much, much broader than that."
General Sengelman said the inquiry was built on a foundation of "sadness, sad stories, terrible things" and traumatic events.
"I've got some inside of me," he said.
"It's all there.
"There's lessons learnt that perhaps shouldn't have been learned.
"Did I make mistakes as a leader, and in hindsight, do I regret them?
"Yes, yes."
General Sengelman spoke about his experience transitioning to civilian life, with a medical appointment leaving him feeling "rubbish".
He also suggested considering the merits of restorative justice, as there was a "lot of hurt and pain out there".
"Every story we know affects a lot of people," he said.
Defence Minister Richard Marles and Veterans' Affairs and Defence Personnel Minister Matt Keogh are giving evidence on Thursday.
The royal commission is holding its final public hearings this month, with a final report to be handed down in early September.
The inquiry has heard from the family members of serving personnel who died by suicide.
In response to the evidence given on Wednesday by a navy veteran, royal commission chair Nick Kaldas said he found it disappointing and concerning the events happened while the royal commission was taking place.
Mr Kaldas said the actions of some of the senior people within the Australian Defence Force were carried out with an "air of impunity" with nothing being done to hold anyone accountable.
An interim report released by the royal commission in August 2022 made 13 urgent recommendations to the federal government.
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