A mystery fungus linked to Ireland's potato famine is killing kauri trees.
Scientists say the exotic fungus is the same one discovered on Great Barrier Island in the 1970s but don't know how it reached two of the country's showcase kauri stands, the Waitakere Ranges and Northland's Trounson Kauri Park.
Although mainly younger trees are affected, some sick trees are up to 100 years old. At Maungaroa Ridge, above Piha Beach, one patch of dead or dying kauri spreads over at least 2ha.
"The fungus causes dead patches or lesions which then ooze a lot of gum and these essentially creep round the base of the trunk and ring-bark the tree," said Landcare researcher Dr Ross Beever.
"I think there's reason to be concerned, certainly there are some [big trees] around the Cascades where we find these large lesions."
The fungus belongs to the Phytophthora genus, a different species of which devastated Ireland's potato crop in the mid-1800s, leaving thousands of people to starve.
"Phytophthora really are fantastically difficult pathogens but this fungus is not known anywhere else in the world," said Dr Beever.
"I suspect it's come in from somewhere because it's too damaging to kauri to have been here long."
Dr Beever faces at least another year's research investigating how lethal the fungus has been on Great Barrier since its tentative identification in 1974.
"I have visited the site and it seems to have increased significantly since then. We are trying to get aerial photos to see what it's doing now."
West Auckland entomologist and Forest and Bird president Dr Peter Maddison said the success of the fungus at Piha could mean the trees were already under stress.
"That could be a sign of the global warming effect," he said. "While the dying trees are not the giants, there are big trees that are dying."
He thought it unlikely the fungus had somehow transferred from Great Barrier.
The fungus has also attacked a handful of other species at Piha, including kanuka and northern rata.