SYDNEY (AFP) - Australian officials on Sunday blamed a "fairly major breakdown" in border security for the reported escape of a convicted terrorist through Sydney Airport to join the conflict in Syria.

New South Wales Premier Barry O'Farrell expressed concern after media reports that Khaled Sharrouf, who served almost four years in prison after pleading guilty over a 2005 conspiracy to attack Sydney, had fled the country.

According to News Corp Australia, Sharrouf boarded a flight to Malaysia at Sydney Airport last month using his brother's passport and was now believed to be in Syria.

"I have to say I think that immigration and the federal police and customs have been doing a magnificent job," Mr O'Farrell told reporters when asked about the case.

"But I look to see what caused what appears to have been a fairly major breakdown."

Police and customs have refused to comment on the case.

Sharrouf, 31, had his passport confiscated and has been on international watchlists since his release from prison in 2009.

He pleaded guilty to committing acts in preparation for a terrorist act by possessing clocks and batteries to be used in a bomb blast as part of the so-called "Terror Nine" conspiracy, which resulted in Australia's largest-ever terrorism trial and the conviction of nine men.

Sharrouf served three years and 11 months in jail for his part in the plot.

Australia's government expressed concern last month at the growing numbers of its citizens travelling to Syria to fight alongside rebel groups, with several reported deaths.

Attorney-General George Brandis said he was concerned about Australians returning radicalised and with new skills to commit extremist acts after fighting in Syria, where a three-year civil war has left more than 130,000 dead.