The Australian government is unable to confirm if Islamic State fighter Khaled Sharrouf is alive but it is concerned about media reports he could be.

The Australian, who moved with his children to Syria to fight with the terrorist group, was believed to have been killed in a drone strike last year.

But someone claiming to be the convicted terrorist has made threatening calls to people in Sydney over attempts by the NSW Crime Commission to seize his house, News Corp reports.

The federal government is unable to confirm whether Sharrouf is alive, Foreign Minister Julie Bishop said in a statement on Sunday.

However, Ms Bishop said she was disturbed by the media reports, which come a week after it was reported his wife Tara Nettleton had died from complications from appendicitis.

"Mr Sharrouf joined the violent terrorist organisation Daesh in Syria and Iraq, and recklessly brought his wife and five children into a conflict zone controlled by the group," Ms Bishop said.

"If Mr Sharrouf is alive it is clear that he continues to show the same lack of concern for human life, including that of his family."

Ms Bishop said reports of Ms Nettleton's death raise serious concerns about the welfare of the children.

Deputy opposition leader Tanya Plibersek has described Sharrouf as a "repugnant" man and one of the worst people to leave Australia to fight with Islamic State.

"I don't think people shed a tear when the initial reports of his death came through," she told reporters in Sydney on Sunday.

"I think these most recent reports, if they are in fact true, just show that he's even more despicable than we thought."

Ms Nettleton's mother Karen has appealed to the Australian government to do everything possible to extract the couple's five children and a grandchild stranded in Syria.