Sharrouf granny in mortal danger

Prime Minister Malcolm Turnbull says he understands the "human dimension" to a Sydney grandmother's rescue mission to save her five grandchildren in Syria.

But he maintains there are good reasons for laws banning Australians from travelling to Syrian trouble spots.

Karen Nettleton and solicitor Robert van Aalt reportedly departed Australia on Thursday night to journey to the Islamic State stronghold of Raqqa, where it's believed the orphaned children of IS fighter Khaled Sharrouf are living.

Foreign Minister Julie Bishop confirmed the government had been in contact with representatives of the family.

"This is a very tragic case," she told reporters in Sydney.

There was very limited consular capacity in Syria and no embassy in Damascus, Ms Bishop said.

"The message that we have been giving is that Australian citizens must not travel to Syria and Iraq unless they have a lawful reason for being there," she said.

People ignoring that advice were putting "their own lives in mortal danger".

"Everybody understands the human dimension to this," Mr Turnbull told reporters.

Sharrouf moved with Ms Nettleton's daughter, Tara, and their children to Syria to fight with the terrorist group.

He is believed to have died in a drone strike last year, while Tara Nettleton is thought to have died in Syria in September after problems arising from an appendix operation.

Their 15-year-old daughter, Zynab is likely to have been left as the family elder to her siblings - aged five, 10, 11 and 13, as well as her own baby daughter - in violent Raqqa.

The children have reportedly texted their grandmother saying they can't escape because of airstrikes.

Experts have warned Mrs Nettleton and Mr van Aalt they could be on a suicide mission.