The case against Melbourne man Jamie Reece Williams, charged in 2015 with attempting to travel to northern Iraq to fight with Kurdish forces against Islamic State (IS) militants, has been dropped.

Key points: The Federal Attorney-General decided to discontinue the case against Jamie Reece Williams

The Federal Attorney-General decided to discontinue the case against Jamie Reece Williams Mr Williams said he was very happy with the result

Mr Williams said he was very happy with the result The move could have ramifications for other Australians who have fought against IS

The decision to discontinue the prosecution was made by Federal Attorney-General George Brandis.

A spokesperson for the Attorney-General said the prosecution of Mr Williams required the Senator's consent.

"In considering giving his consent, the Attorney-General has a broad discretion and is able to take into account a number of factors," the spokesperson said.

"On this particular occasion, the Attorney-General did not consent to the prosecution. It would be inappropriate to comment further."

The move could have ramifications for Australians who have fought against IS, returned to Australia, but are still under investigation by the Australian Federal Police (AFP).

Mr Williams, of Epping, in Melbourne's northern suburbs, was charged in July last year after being detained at Melbourne Airport in 2014 as he attempted to board a flight to the Middle East.

When asked what he was travelling for, Mr Williams told authorities he was going to fight with a Kurdish militia called the YPG, and planned to travel first to the northern Iraqi city of Sulaymaniyah.

Late last year Mr Williams' lawyers applied to have the case against their client discontinued by the Commonwealth Director of Public Prosecutions (CDPP), arguing that the prosecution was not in the public interest because the Kurds are an ally of the coalition fighting against IS.

They also said the YPG was in fact the effective government of parts of northern Syria.

Under the foreign fighters legislation, which Mr Williams was charged under, anyone who fought with or planned to fight with the armed forces of a government was exempt from prosecution.

Mr Williams' lawyer Jessie Smith argued that the YPG and its political wing exerted "effective control" over parts of Syria, and therefore constituted a government under the act.

It is not clear on what grounds the CDPP has decided to discontinue the prosecution.

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Williams 'very happy' with decision

Mr Williams walked from the County Court supported by his girlfriend Jessica and Queensland man Ashley Dyball.

Mr Dyball returned from Syria in December last year after spending six months fighting with the YPG.

"I'm very happy with the result, that's all I have to say," Mr Williams said to media outside the court.

The AFP have been ordered to pay Mr Williams' legal costs.

The decision raises questions about the police investigations into Matthew Gardiner and Mr Dyball, who have both returned to Australia after spending time with Kurdish forces fighting against IS.

Mr Gardiner, a former Northern Territory Labor Party president, spent three months with Kurdish groups last year and returned to Australian in April.

A registered nurse, he was questioned for several hours by AFP officers on his return to Darwin, but not spoken publicly about his activities in Syria.

Under the foreign fighters legislation, Mr Gardiner faces life in prison if found guilty.

Mr Dyball returned from Syria in December last year after spending more than six months with the YPG.

He was also interviewed for several hours by the AFP and released without charge after he was deported from Germany.

The AFP continue to investigate both Mr Dyball and Mr Gardiner.

Last week Canadian man Robert Somerville, who was deported from Brisbane over his links to Kurdish forces, told the ABC that he was asked questions about Mr Dyball's activities in Syria.

Counter-terrorism expert Greg Barton said it would be premature for Matthew Gardiner and Ashley Dyball to be celebrating today's decision.

"If you were in the defence team you would be feeling encouraged but I don't think it necessarily has direct ramifications," Professor Barton said.

"Whatever they do [Mr Dyball and Gardiner] shouldn't be gloating in public because they have got a long way to go, in private they might feel a little bit encouraged but their cases are very different given they've been picked up going back.

"It's clear they were in the region, they may make an argument about what they were doing but they are in a very different scenario."