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," O'Farrell told reporters when asked about the case.
Police and customs have refused to comment on the case.
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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.
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.