Australia's Islamic State kingpin Mohammad Ali Baryalei was a "clean cut young man" when he lived with his mother in Quakers Hill, neighbours say, but something changed before he flew to the Middle East.

The terrorist, who is believed to have risen to become the most senior Australian in Islamic State, has reportedly been killed in Syria, where he had recently taken up arms for the militants.

Neighbours said that they noticed a change in Baryalei, a former Kings Cross bouncer, about a year before he went to fight in Syria, in 2013.

"When he started growing that beard he became really serious. He was much more light-hearted before then," they said.

"He left the house at 17, after his father kicked him out. But he would still help out around the neighbourhood. If fences needed to be fixed or if there were other odd jobs that needed doing, Ali would be there.

"He was a clean cut young man. Somebody must have gotten inside his head."

Homira Baryalei was seen leaving her home on Wednesday morning, as reports of her son's death circulated. The government was still trying to confirm his death on Wednesday evening.

Mrs Baryalei's neighbours, who asked not to be identified, noticed nothing unusual at the home on Wednesday.

"She waved to us, nobody seemed terribly upset," they said.

But Baryalei's mother had been distraught when she found out her son had gone to fight for Islamic State.

"When she first found out she was in denial. Then it dawned on her.

"She's mostly on her own; she comes over and chats all the time. We used to call her Hail Mary."

Baryalei, 33, had been situated on the Turkish-Syrian border since he left Australia in mid-2013, receiving new fighters from Australia and facilitating their passage into Syria to join terrorist groups.

It appeared he had moved into Syria recently and taken up arms, according to photos that surfaced online.

His mother's neighbours said that up until a year ago he would check in with his mother.

"He used to call every month, then after the raids he stopped ringing all together."

with Rachel Olding and David Wroe