"Someone had thrown it, with great force I imagine, off the bridge.
"My partner swerved to miss it, then it rebounded off the road.
"He was pretty shaken by it, it could have literally killed us."
O'Donnell reported the incident to police after posting about it on Facebook.
"Hey can we like not throw electric scooters off bridges into on coming traffic because someone just did exactly that outside of the Terrace Tunnel & it was cm away from going through our windscreen and we had to swerve to miss it, thanks," she wrote.
Other Facebook users commented, urging her to report it.
"Police would like to remind those responsible that throwing any items from bridges poses serious risks to vehicles and their occupants," a police spokesperson said.
"Matters like this are taken extremely seriously.
"Motorists are urged to remain vigilant and call 111 if they spot suspicious activity taking place."
O'Donnell said the scooter was owned by Flamingo and after she spoke to police the company reached out to her and said they were also investigating.
A woman travelling behind O'Donnell - who had three young children in the car - also contacted her via Facebook and said she witnessed the horror incident.
"It was really scary, we were all fine but it could have gone through our window, my partner could have crashed into the concrete wall."
O'Donnell said the person who threw the scooter was "an idiot" and she urged them not to behave so stupidly in future.
In 2005 Taupo man Christopher Currie, 20 was killed when an 8kg block of concrete was deliberately dropped from an Auckland motorway overbridge and smashed through the windscreen of the car he was driving.
Ngatai Reweti, then 14, was jailed for nearly four years for manslaughter.
He was released on parole in March 2009.
In 2011 Auckland woman Ana Leo came close to tragedy when a large rock was thrown from an overbridge and crashed onto her car's bonnet, shattering her windscreen.
Ana Leo was driving on the Southwestern Motorway at 10.30pm on a Thursday night when the rock landed on her Ford Territory.
As well as rocks, traffic cones have been thrown off the bridge.