One woman is dead and two people are in a critical condition after a tourist bus plunged off a cliff on the Great Ocean Road.

Police said they believe the bus, which was carrying seven people, crashed 20 metres down a ravine near the intersection of The Boulevard at Gellibrand Lower around 2pm on Thursday.

The female passenger died at the scene.

Police said the bus appeared to have been carrying adult Korean tourists.

Emergency services worked for about an hour to free four trapped passengers from the wreckage, an Ambulance Victoria spokeswoman said.