RCMP say the incident happened just after 3 a.m. PT when a group of teens was trying to cross the highway near Boys Road.
First, the 13-year-old girl was struck by a vehicle, leaving her in pain but still conscious. The driver remained at the scene.
But while waiting for police and ambulance to arrive, the girl and an 18-year-old male were struck by a second vehicle that was making a lane change to avoid the first vehicle.
The girl was pronounced dead at the scene. The 18-year-old was taken to Cowichan District Hospital with significant injuries.
Police say alcohol and speed are not believed to have been factors in the collisions.
Mounties are hoping other teens who may have left before police arrived will come forward.