Kauri dieback infection has doubled in the Waitakere Ranges in the last five years. Unless we change what we're doing now, say the biosecurity and parks managers of Auckland Council, all the kauri in that forest will be infected within 30 years. And because there is, as yet, no known cure, all those trees will die.

Councillor John Watson told the council yesterday that if kauri dieback was a human pathogen, we would treat it exclusively as a biosecurity issue and close off the entire area right now. He thought we should adopt that approach with kauri dieback.

His colleagues listened politely but did not agree. They adopted a more workable approach.

Still, what they've agreed is a very big thing: the biggest closure of a public park in New Zealand history, they were told.

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There had three reasons to take drastic action. One was to try to preserve the inherent value of the Waitakere forest: its lifeforce, its mauri. We are, all of us, the kaitiaki of that forest. Its guardians.

The second was that there are still important stands of kauri in those ranges that are not infected. It's possible they can be saved.

The third was that the rest of our forests can be saved. Kauri dieback has not yet infected many of Auckland's other forests and the council wants to keep it that way.

The key to doing that is to stop people tramping infected soil out of Waitakere forest and into other areas.

The council's decision marks a significant step forward in its relationship with mana whenua. Last year the local iwi, Te Kawerau a Maki, set up a rahui that put the park off limits. But as far as the iwi was concerned, the council ignored it.

Since then both sides have worked hard, and with goodwill, to find common ground. Their success will be evident on April 19 and 20, when experts from both sides walk the 13 or so tracks whose status is still unclear. They'll decide together if those tracks are ready to reopen, or need more work, or should stay closed.

A practical partnership and a quite profoundly symbolic moment too.

The council took another brave step with this decision. In the public consultation process it received over 800 individual pieces of feedback on its proposals, with 43 per cent of respondents wanting more tracks kept open. Many politicians might have been scared off by that. But a combined 48 per cent was in favour of the proposed track closures or wanted even more.

In the end, it wasn't about those numbers. Councillors spoke of their responsibility to the future. Almost unanimously, they did what they believed was right, not for any political advantage but because they believed it was right. You don't see that a lot.

There is still much to resolve. Will the small number of tracks that stay open cope with the increased traffic? If a "safe" track is a boardwalk or a boxed gravel path, how do you offer a wilderness experience? How many of us will accept that we should choose other forests – especially non-kauri forests – to walk and run and ride in? Will the containment measures work?

It's a work in progress. But it's a very good work to have in progress.