The first Australian to be charged with attempting to fight against the Islamic State (IS) group says the failed prosecution against him could encourage other Australians to join Kurdish forces in the fight against the terror group.

Key points: Mr Williams thinks his case may increase support for the Kurdish cause

Mr Williams thinks his case may increase support for the Kurdish cause He thinks people were waiting on the result of his case before deciding to fight

He thinks people were waiting on the result of his case before deciding to fight He's called on the Government to clarify Australia's foreign fighters law

Jamie Reece Williams was charged under foreign fighters legislation after he was stopped at Melbourne Airport in December 2014 while trying to travel to the Middle East to join the YPG, a Kurdish militia battling IS in northern Syria.

This week, Federal Attorney General George Brandis instructed the Commonwealth Director of Public Prosecutions to drop the case against the Melbourne man.

"I think it is ridiculous to be honest. The Kurds are an ally of Australia. ISIS are an enemy of the world. For somebody to be prosecuted for trying to do something — whatever little they can — about this is absurd to me," Mr Williams told the ABC in his first interview since he was charged.

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"If anything there is probably more attention and more support for the Kurdish cause because of this case than if they were trying to dissuade people from going and getting involved.

"I am sure there probably are a lot of people that were waiting to see with my case whether they will go or not."

Grey area of foreign fighters law needs clarification

Mr Williams said it was alarming that Australian Federal Police who questioned him had been uncertain whether he had broken the law or not, and that the Government must clarify whether Australians trying to join anti-IS forces would still be prosecuted.

"It was frustrating to be honest. This is their job to know these things, they are the ones enforcing the laws. It was explained that if I had have been a month earlier they probably wouldn't have been able to stop me," he said.

"But considering the new legislation that was brought in, it was very grey and difficult to understand. And reading over the legislation myself I can see how it is confusing."

The reasons for a decision to stop the prosecution are still unclear, with Senator Brandis refusing to comment on the case this week beyond a brief statement saying he had exercised his "discretion".

Other Australians who served with the Kurds in limbo

Mr Williams' lawyer, Jessie Smith of Melbourne firm Stary Norton Halphen, argued in preliminary hearings that the prosecution should be discontinued on two grounds: that it was not in the public interest (because the Kurds are allied to Australia in the fight against IS), and that the YPG is the de facto government of part of northern Syria, which, under the legislation, would clear Australians to fight with the militia.

Senator Brandis' refusal to explain on which of these grounds he made his decision has left other Australians who served with the Kurds against IS and returned home in limbo.

These include Queensland man Ashley Dyball — who supported Mr Williams in court this week — and Northern Territory Labor Party figure Matthew Gardiner.

Both men are under investigation by the AFP and have not spoken publicly about their activities in Syria.

Dr Nicola McGarrity, director of the terrorism law reform project at the University of NSW, said the decision to drop the prosecution indicated that Mr Dyball and Mr Gardiner could be safe from prosecution.

"I think the strong implication is that they probably will not be prosecuted. Even in relation to Jamie Williams it seems to be something of an afterthought to prosecute him," Dr McGarrity said.

"He was stopped at the airport in December 2014 from getting on a plane to Qatar and yet it wasn't until eight months later that he was actually arrested on these foreign incursions charges, and now what we've seen six months after that is that the prosecution has been dropped altogether."

Despite the ambiguity around Senator Brandis' decision, Mr Williams said he felt the dropped prosecution was a vindication of Reece Harding and Ashley Johnston, two Australians killed while fighting with the YPG.

Ashley Johnston and Reece Harding were both killed fighting against Islamic State. ( Supplied )

"I think it is good for the families to have some closure, that they're not considered criminals anymore, that they gave their lives for a good cause. The most offensive part about it was the Government's stance that they were breaking a law by doing that," he said.

Freed from the burden of his legal case, Mr Williams said he was considering whether he would now travel to Syria and join the fight against IS, more than a year after he first tried.

"Obviously I don't want to end up in this situation again. I have to abide by Australian law," he said.

"Whether this is a green light for people to go and join the YPG is still uncertain at the moment. Charges were withdrawn but it is still unclear the reason for that. I think there needs to be some sort of explanation as to how the law sits now."