A rare flower has been found in Mount Remarkable National Park in South Australia.

A spider orchid had not been seen in the park since 1978, until a walker found one recently near Melrose.

Southern Flinders district ranger Danny Doyle says the discovery underpins the park's conservation value.

"It's a small white flower, about an inch across, single leaf at the base with a slender stem of about 20 centimetres high," he said.

"I guess when the flower's not there it's fairly nondescript but once you see the flower, it's quite a distinctive white flower, which is quite unusual."

He says conservation of orchids was one of the main reasons the national park was established in 1972.