The 17-year-old was on top of a red and white mini-Cooper sedan with three friends as it drove around the Esplanade and Davies Street in Altona at about 11.30pm.
It’s believed the group were celebrating the Samoan national side’s recent victory at the rugby league World Cup.
The 17-year-old Melton South woman fell off the car as it turned and struck her head on the ground at about 11.30pm.
She is receiving urgent care in hospital.
Another teenager has since arrived at hospital who was also injured in the same incident.
Police arrested a 21-year-old Braybrook man in Altona this afternoon, while a 21-year-old Caroline Springs man presented to a police station shortly after.
Detectives subsequently seized a mini-Cooper sedan.
Police said both men are currently assisting police with enquiries.