Kauri could be wiped out of the Waitakere Ranges by dieback disease within three decades if drastic action isn't taken, it's been claimed.
"It's easily the worst pest threat I've ever encountered," former Auckland Council biosecurity manager Jack Craw told The AM Show.
"It's death for every tree that's infected. There is no cure."
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Mr Craw, who has almost 40 years' experience in biosecurity, says it can take 10 years for symptoms to show after a tree's been infected, meaning the disease could be more widespread than we know.
"We simply can't cure this disease. It gets in the roots and kills all the roots off and causes the tree to die by bleeding to death."
Last month a rahui was placed on the Waitakere Ranges by Te Kawerau a Maki, in the hope of halting the spread. Auckland Council responded by closing even more tracks.
Mr Craw says some of those could stay closed for good.
"It doesn't mean everyone's out forever. But some of those tracks should be closed for a very long time and some should be closed permanently.
"We don't know how long that disease lives in the soil. We suspect 40 years."
There's no simple cure to the disease, caused by a microorganism similar to a fungus, because most fixes would risk "killing all of the other microbiota around the trees".
"I'd say unless something drastic is done we won't have any kauri in the Waitakere in 30 years."
Mr Craw says drastic action needs to be taken now - and it won't break the bank.
"It would take $8 million per year. That's not a lot of money to spend... it's chicken feed.
"But remember you can't bring back the dead. We really do have to have rapid action now."