Not all flowers smell like roses, but a new plant discovered in the Blue Mountains sprouts blooms that smell like rotting fish.

Botanist Greg Steenbeeke was one of three scientists to identify the new species in a small patch of rainforest in the Megalong Valley.

A new species of flower discovered in the Blue Mountains smells of rotting fish. Credit:Greg Steenbeeke

"I've gone past the point where we found the plant several times a year for two decades and never knew it was there," said Mr Steenbeeke, the NSW Office of Environment and Heritage senior threatened species officer.

Despite its wafty odour, the plant, botanical name Thismia megalongensis, is hard to spy – its small orange flowers grow on the forest floor and are smaller than a five cent piece.