A man who's spent 20 years trying to validate a wartime conspiracy theory says he's cracked the case.

A commercial pilot who's spent the past 20 years trying to validate a popular wartime conspiracy thinks he may have struck gold, or, in this case, tunnels.

North Head, a volcano and historic fort at the mouth of Auckland Harbour, has long been the centre of tales about hidden aircraft from World War I, sealed-off tunnels and expired ammunition.

But multiple court cases and investigations in the past 30 years deemed those to be no more than urban myths.

After paying for ground penetration radar (GPR) research from his own pocket, Martin Butler thinks he's landed on evidence that opens the lid on the case.

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"I did have serious doubts when we started this ground radar," Butler said, "[Now] I have no doubt in my mind we will find underground structures at North Head."

Alexander Turnbull Library Two members of the Women's Army Auxiliary Corps operating a plotting table during one of the World Wars. Butler believes the command centre is part of the features illuminated by the ground penetrating radar.

The Auckland resident, who said he was not the sort of person who "jumped at flights of fancy", became interested in the conspiracy after an early 90s DOC investigation and subsequent court case decided against the existence of tunnels.

Butler wasn't convinced, pointing out 50 individual sworn statements and a 1989 letter from the navy Commodore said otherwise.

In 2012, he decided to shift his own investigation from eye-witness accounts to a scientific approach and hired the services of ScanTec Geophysical Consultants, a company whose portfolio includes work on the newly built Waterview tunnel.

Matthew Rosenberg Martin Butler has spent over 20 years researching North Head.

Four years later, a letter from the company's director Matt Watson said the results of their research had uncovered "a large number of features that are clearly man-made, and have the characteristics of tunnels".

Watson said their GPR findings included straight lines and radial patterns "clearly related to modern underground workings".

The features were not linked to the current open network of tunnels, indicating they had been blocked at some point in the past.

Sir George Grey Special Collecti Boeing Bluebill c.1918 taking off directly towards North Head with Mount Victoria visible to the left. Butler believes there is a possibility the plane lies within North Head.

For Vernon Rule, an ex-Royal New Zealand Navy petty officer who gave sworn testimony about the existence of extra tunnels, Butler's radar results hadn't come as a surprise.

Along with friends, he had spent many lunch hours navigating the intricate tunnel system with ropes and torches but said the place was almost unrecognisable now.

"Some of what we went through has been covered over and is just not accessible," Rule said.

Supplied Results from the radar testing show radial patterns and straight lines, the result of man-made structures according to the company who conducted the research.

"The whole look of the place has been transformed."

Rule believed ammunition was still inside the hill as the result of a "mutiny" that happened during his time in service when soldiers removed some of the shells from inside the mountain but refused to complete the job.

In 2012, Butler received a response from then-Defence Minister Jonathan Coleman saying the Royal New Zealand Defence Forces had informed him potential decaying ammunition didn't pose a risk to public safety.

Stuff David Veart is a writer, archaeologist and historian. He was also the former DOC investigator in charge of the 1990 - 1992 investigation. He has seen the radar evidence and is not swayed by it.

But Butler remained unconvinced.

"There is little doubt, if there is munition there, that one day it will explode," he said.

"It's not if, it's when."

Sir George Grey Special Collecti North Head, Devonport. Butler believes this photo shows a concealed tunnel entrance Ref: 4-3001.

DOC remained firm on its early 90s stance, saying the radar search produced no clear evidence of tunnels or "anything else hidden in North Head".

"DOC spent two years, from 1992 till 1994, looking for the hidden tunnels. We found no trace of any hidden tunnels containing ammunition and airplanes," DOC Auckland mainland operations manager Kirsty Prior said.

The archeologist in charge of that investigation, David Veart, had seen the radar results but didn't believe it proved Butler's theories.

"My stance is pretty much as it was when we finished the excavations in the mid-90s," Veart said.

"There's no convincing evidence, either physical, photographic or historical, that I can find, for any major tunnel system over and above those ones you can see today."

Butler has written a book on his 20-year efforts - Tunnel Vision, inspired by his concern about deteriorating munitions.

"We may be putting future generations at risk."