"Our inquiries so far have determined the projectile has come from a high-powered rifle in the nearby area," Acting Detective Senior Sergeant Nick Salter said.
"However, we would like to reassure the public that we do not believe the vehicle was specifically targeted. Analysis suggests the bullet has ricocheted through the windscreen."
"There were a number of vehicles travelling on the motorway at that time and we would like to hear from any drivers that witnessed the incident," Salter said.
"We would also like to hear from anyone who was on the eastern side of State Highway 16 and in the Royal Rd area who may have heard a loud bang or noise around the time of this incident."
Following the incident a small hole could be seen in the bottom corner of the windscreen of the car the woman was in.
Residents from homes backing on to the motorway said they heard a loud bang. One thought it sounded like a gunshot. Another described it as a "loud crack".
Anyone who witnessed any suspicious behaviour in the area of the incident should get in touch with Henderson Police by phoning the 105 number, police said.
Information can also be given anonymously through Crimestoppers on 0800 555 111.