The aerial survey that has covered more than three million hectares looking for kauri dieback - finding many sites in Northland that need further investigation - is about to go to ground.

The aerial surveillance has involved flying at low level, taking nearly a million photographs and covering around 40,000km - equivalent to one circuit of the Earth.

The next step is likely to be ''ground-truthing'' — getting down on the ground to find out why a tree looks sick.

But as a result of that work in the air, 450 kauri sites across Northland, Auckland and the Waikato have been identified for possible investigation.

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Those sites exclude the Waipoua Forest where the management partnership between local iwi Te Roroa and the Department of Conservation (DoC) is undertaking its own kauri dieback programme.

The Aupouri Peninsula and Kaitaia areas are expected to be surveyed within the next few weeks, with few, if any, sites expected there.

Northland Regional Council (NRC) admits it has a "huge task" ahead with the ground-truthing programme.

The council will work closely with landowners. Under the Northland Regional Pest Management Plan any suspected kauri dieback must be reported by the landowner to an appropriate management agency.

"We've been working with them to develop personalised kauri dieback disease management plans to try to reduce the risk of the disease spreading from private land and district council reserves and will continue to do so,'' NRC Environmental Services group manager Bruce Howse said.

John Sanson, Ministry for Primary Industries (MPI) manager of recovery and pest management, said the next step in the national programme — the ground-truthing — will require MPI, DoC and regional councils to prioritise sites to visit.

"One of the symptoms of kauri dieback is a yellowing and thinning canopy. This may also be caused by other things such as drought, poor soil conditions, high winds, cattle and other animal movement under the tree,'' Sanson said.

Ground-truthing is a time-consuming and costly process but for now is the best way to verify whether kauri dieback disease is actually present, he said.

Around 75 per cent of the identified 450 sites are individual trees in bush areas, the majority in Whangarei/southern Northland, southern Kaipara and Rodney. About 50 are in the Waikato region.

Up to $180,000 has been spent by the national Kauri Dieback Programme each year since July 2015 on surveillance, GIS mapping and data input, and laboratory analysis of samples.

It's not all about sick trees though; each photograph taken is also assessed to identify the number of kauri trees and their health.