Key Points:

A new disease is slowly spreading through the Waitakere Ranges attacking and sometimes killing kauri trees.

The Auckland Regional Council is worried about the disease, called kauri collar rot, and wants the public to take precautions like scrubbing their shoes before and after entering the ranges.

Sandra Coney, chairwoman of the ARC's parks and heritage committee, said she was concerned the disease was hitting trees of all ages, including large kauri hundreds of years old.

Ms Coney said the forest had been recovering in the past century after a long period of timber milling.

"It would be completely tragic if the disease got out of control and devastated large areas."

Ms Coney said hundreds of trees were already dying or dead but the extent of the disease spread was not known.

To date it had been found in Piha, Huia and the Cascades Kauri Park.

Ms Coney said the disease was first discovered along the Maungaroa Ridge near Piha in 2006 but it was not known what was causing the symptoms or if it would kill the trees.

Kauri dieback caused by a phytophthora was first discovered on Great Barrier Island in the 1970s.

After extensive research by Landcare Research it was now confirmed as a deadly new form of phytophthora.

The soil pathogen known as phytophthora taxon agathis caused a progressive collar rot that could girdle the tree and eventually kill it.

Affected trees show yellowing leaves, thinning canopy and dead branches.

The trees could also develop lesions that bled resin across the lower part of the trunk.

Ms Coney said the pathogen could be spread by soil or water and pigs were a potential vector.

Pigs were already a problem in the ranges and this gave an added urgency to have them destroyed by contract hunters.

Ms Coney also urged people taking dogs into the ranges to keep them on a leash and not allow them to forage.

The council would be asking the public to clean their footwear and for event organisers to take extra measures like disinfectant footwear mats or footbaths.

Already the organisers of a running race scheduled for Piha early next month had decided to relocate the event after the council said it would not be able to go ahead should there be heavy rain which might help spread the disease.

Ms Coney said the regional council owned the largest area of kauri forest in the Auckland region.

"We need the public to help stop this disease from spreading within the ranges and further afield, and to tell us if they notice sick trees."

More research was needed into the pathogen that specifically attacked kauri.

The council was rolling out standard operating procedures to its staff and contractors working in the ranges as well as communicating with all the relevant organisations and local communities that used them.

The measures were similar to those for biosecurity risks such as didymo in the South Island.

Ms Coney said for other tree diseases spraying or stem injections could help, but common phosphate-based fungicides were not known to be effective against this phytophthora.