A lesson learnt by one tourist in Fiordland National Park.

The 21-year-old woman had gone past the barrier on the Chasm Walk, near Milford, to take a photo, when she slipped and feel 20-meters into the river below.

She managed to swim to a nearby ledge and was rescued about 7pm last night - uninjured but cold.

Police say the woman was extremely lucky and could have died, all for the sake of a photo.