A rare and threatened species of West Coast snail has been captured on camera laying an egg for the first time.

The Powelliphanta augusta snail was being weighed by Department of Conservation (DOC) ranger Lisa Flanagan in when the little egg emerged from its neck.

DOC had been managing a captive population of the snails in chilled containers since 2006, when Solid Energy started mining their habitat on the Mt Augustus ridgeline on the western side of the Stockton Plateau near Westport.

Flanagan said it was a special moment after 12 years looking after the snails.

"It's remarkable that in all the time we've spent caring for the snails, this is the first time we've seen one lay an egg.