The Wollemi pine, dating from the Jurassic period 200 million years ago and often described as a living fossil, was thought to be extinct until 1994 when a park ranger stumbled upon a stand of fewer than 100 trees in a remote gorge in Wollemi National Park, 200 kilometres west of Sydney.
The grove's location has since been kept secret and those authorised to visit it undergo strict infection control procedures.
But the stand has now been infected by the fungus-like disease Phytophthora cinnamomi, which was detected in two trees in November.
Name: Wollemia Nobilis
Discovered by: David Noble, a NSW Parks officer in Wollemi National Park, Australia
Oldest known fossil: 90 million years
Characteristics: Conifer with attractive, unusual dark green foliage, bubbly bark and sprouts multiple trunks
Best use: A stunning patio and indoor plant and feature tree for parks and large gardens
According to John Dengate, a spokesman for the New South Wales environment department, the infection was almost certainly introduced by an unauthorised visitor.
Dengate said: "It is an endangering factor. Nobody can be sure at this stage how big a threat it poses, but we're treating it very, very seriously."
Dengate said the department had begun treatment on the infected trees and was hopeful that the disease could be cured.
Despite the threat, the species is not at risk of extinction because thousands of the trees have been grown in plantations from the wild stand and some went on sale to the public in October.
Dengate exonerated anyone officially involved with the wild trees.
"We've been scrupulous with our staff to make sure they don't carry anything in," he said.
"We understand that people are really keen to see the trees in their natural environment, but there are only a few dozen left and they are extremely vulnerable to infections so we're asking people to stay away."