Amanda Johnston only discovered her son Ashley was at war with Islamic State (IS) in Syria when she learned of his death last week.

Mr Johnston became the first westerner killed fighting for a growing counter-jihadi movement in Syria and Iraq when he was ambushed and shot by IS militants in north-eastern Syria.

Senior security sources told the ABC's 7.30 program Mr Johnston was among three Australians fighting with the Kurdish People's Protection Units (YPG), including former Northern Territory Labor Party president Matthew Gardiner.

A fourth Australian, Khamis Gewargis, is fighting against IS with Assyrian forces in Iraq.

Counter-terrorism authorities believe Johnston, a 28-year-old Canberra postman and Army reservist, was lured to fight with the YPG by offshore recruiters operating on Facebook using the same methods that have proved so effective for IS.

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Ms Johnston told 7.30 that her son revealed he was in the Middle East on December 30, when he sent a text message saying he was doing "humanitarian work" to support the Kurds.

"We asked Ash questions: 'What was he doing? Who is he with?'" she said.

"I said to him, 'You haven't gone and joined some group and you're not doing something you're not telling me?'

"And he would either not answer me or he would just say, 'No, Mum. I haven't, I'm fine', so we had really no idea of what was going on.

"There's still so much that we don't know. We are still finding out things but it wasn't until after he was killed that we found out where he was and who he was with."

Johnston 'killed protecting his comrades'

Mr Johnston had long dreamed of fighting a war.

He trained to join the Australian Army and spent seven years in the Army Reserves, serving for several months as a peacekeeper in Solomon Islands.

The postman and bottle shop manager also tried unsuccessfully to join the Special Forces and the British Royal Marines, before finding his calling late last year with a motley crew of international fighters and Kurdish militia engaged in a fierce fight-back against IS in northern Syria and Iraq.

Mr Johnston's comrades told his mother he was killed while trying to protect them when their tank broke down last week near the besieged town of Tel Hamis.

"They were encircled by ISIS fighters who were closing in and Ash jumped out of the vehicle to, I guess, draw them away and he was shot, but it gave the others in the vehicle the chance to get out and he saved their lives," Ms Johnston said.

"I guess they were able to continue on doing what they needed to do and that's why I know that it's definitely Ash.

"He would put other people in front of his own wellbeing. I guess that actually didn't surprise me.

"The fact where he was and what he was doing and the extent of it, that was really confronting but to know that Ash did what he did, that was devastating, but that didn't surprise me that he would do that for his mates."

Kurds working to bring body home

Mr Johnston is now being mourned as a hero by Kurds in Syria, Iraq and Australia, who are working to bring his body home on a perilous mission through Syria.

Tributes have also flowed from international comrades including Dean Parker, an American surfing instructor who trained with him in Syria and laid a wreath for him at the Australian Embassy in Washington last week.

"I met Ash in Rojava, right on the Rabiah border crossing, one of the command posts," he told 7.30.

"He had come in with a new unit of fighters and we spent about three weeks together there and we learnt our Kurmanji.

"Ash dove right into the language, he would spend an hour practising his Kurdish and then we would practise with the YPG/YPJ fighters.

"He fit right in with the Kurds. Really relaxed, always with a smile on his face, just a really good-hearted, really good human being.

"We were only together for about three weeks but he made a pretty strong impression on me.

"The world needs more people like Ash. Without a doubt, he was truly one of the best people I have ever met."

Kurdish forces pose with IS fighters killed in Johnston's honour

On Facebook, the Kurdish forces hailed Mr Johnston as a martyr and posed with the bodies of IS fighters killed to avenge his death.

The macabre photos alarmed Australian authorities, who warned fighting with groups like the YPG was a crime that carried the same penalties as fighting with IS: up to 20 years in jail under the Foreign Incursions Act.

"They will face the full raft of powers available to law enforcement to deal with it," AFP Assistant Commissioner Neil Gaughan said.

"They're still committing a criminal offence.

"The same issues potentially that arise for someone fighting for Islamic State face someone fighting for the Kurds.

"They're exposed to weapons use, they're exposed to violence, hatred and obviously, I'm concerned about mental health issues when they return."

However, Mr Gaughan suggested Australians fighting against IS could be treated more lightly in the courts.

"It's a very technical piece of law," he said.

"The black and white of it is, yes, they are committing a criminal offence; however, there are some other considerations which need to be taken into account and one of those being that the Commonwealth Director of Public Prosecutions needs to be satisfied that it's in the best public interest to run a prosecution.

"Of course, if the person does go before a court, it's up to a judge or a magistrate to determine the sentence and factors could be taken into account for mitigation of sentence."

The last thing Ms Johnston wants is for her son's death to inspire other would-be fighters, but she insists Mr Johnston was no criminal.

"If I had known that Ash was going to do what he did, I would have done everything I could to stop him going," she said.

"I don't want any other family to go through what we're going through.

"But I believe that Ash didn't believe he was committing a crime. I believe that he would have looked at the moral side of it — that these people needed help and he wanted to help them.

"I think sometimes what's legal and what are morally right can be two different things. Legally, the Government might say he has done the wrong thing and he is a criminal but I would not class him as that.

"I think that morally, he's done what he thinks was right."