Never-before-seen photos of the deadliest US volcano eruption have been discovered from an old camera purchased an op shop for $20.

38-year-old Kati Dimoff purchased the vintage camera from a Goodwill shop and developed a roll of film that was inside.

Kati was shocked to find the camera had photos of the Mount St Helens 1980 eruption which killed 57 people and caused over $1.1 billion in property damage.

The eruption was triggered by a magnitude five-plus earthquake.

It is not certain if the photos were documented on the day of the eruption or after the main eruption had subsided.

After Kati shared the images online, a family identified the people that were pictured in one of the photos.

The previous owner of the camera was identified as Faye Gardner, who passed away in 1981.

She was the grandmother of a man named Mel Purvis, who spotted himself in the photos with his wife and their late son while on the internet.

He said he nearly “fell off his chair” when he came across the photos.

Kati plans to return the camera to Mel. Mel sees it as a remarkable reunion with family memorabilia as his mother recently passed away.

Scroll through the gallery above to see the photos from the vintage camera.