The girl is believed to have been in year seven at Sirius College in Broadmeadows.
Australian Foreign Affairs Minister Julie Bishop said the Australian government was providing consular support to her family.
"I extend our deepest sympathies to her family, her loved ones, her fellow students in Broadmeadows," Bishop told reporters in Canberra.
"This tragedy underscores the brutality of this terrorist organisation that shows no respect for religion, nationality, sovereignty, borders, no respect for humanity."
A closed-circuit camera captured the moment of the blast, showing a busy downtown avenue with cars driving down the street.
A huge fireball then engulfs a building as drivers scramble to get away.
Other videos of the attack posted on social media show wounded and bloodied people crying for help on the footpath outside the ice-cream parlour.
There are no reports any of her family members were injured in the blast. A funeral will reportedly be held today.
In a second attack, an explosives-laden car went off during rush hour near the state-run Public Pension Office in Baghdad's busy Shawaka area, killing 14 and wounding at least 37 people, a police officer said.
Isis claimed responsibility for the two attacks, in statements released online, saying its suicide bombers had targeted gatherings of Shia Muslims.
The attacks happened just days into the holy month of Ramadan when people of the Islamic faith fast during daylight hours.
- AAP, AP
