A test treatment for kauri dieback has seen some dramatic results, but scientists warn that getting the dose right is still critical.
The fungus-like disease, which affects trees of all ages, is killing kauri forests in the upper North Island, and a major infection west of Auckland could see the Waitākere Ranges Regional Park closed to the public.
Microscopic spores infect the trees' roots, stopping them from absorbing nutrients and water, and eventually starving the trees.
But Ian Horner from Plant and Food Research has led a five-year trial of the common horticultural chemical phosphite, to treat kauri suffering from phytophthora fungal disease.
He said in virtually every tree that was treated, the lesions on the trunk stopped bleeding and dried up - but some trees were damaged by the high levels of phosphite.
Dr Horner said it was now just a matter of fine-tuning the dose needed to stop the rot, without hurting the tree.
"Because we're still not certain about what is the correct dosage to use, and that's what this second round of trials is about. We don't want to end up with people killing trees by having the dose wrong.
"Although the counter-argument to that is the trees are probably going to die anyway if they've got kauri dieback, because that seems to be the prognosis in most cases."
Dr Horner said people in Auckland and Northland had been helping Plant and Food Research test phosphite and he would welcome more offers of help from other landowners with kauri on their properties.