Simpson pulled her vehicle to the side of the road when heavy rains began to fall.
"I wasn't driving very fast because I couldn't see very well," Simpson told the Australian Broadcasting Company. "I couldn't see in front of me, I couldn't even see the line on the road."
Simpson said she then heard a loud bang.
"All this rain starts coming in, and the back window where my daughter was was just open ... it's gone," she told the ABC.
Simpson jumped into the back seat of the car to protect young Clara.
On Facebook, Simpson described covering her baby and suffering bruises on her back, arms and head.
After a front window was smashed, Simpson moved to help her grandmother but both women were pelted by the stones, the ABC reported.
When the storm passed, Simpson drove her damaged car to a nearby home, where the residents called an ambulance, the ABC reported. Simpson's grandmother spent the night in the hospital after she suffered cuts to her left arm while her right arm was left "completely black" from bruising, the television station reported.
Clara suffered minor injuries.
Simpson posted Friday on Facebook that she and Clara were “safe at home,” while her grandmother was recovering in the hospital and would be home soon.
“Thank you all for your concern,” Simpson wrote, adding that she had received hundreds of messages, friend requests and telephone calls.