The historic World War II tunnels at the Stony Batter Historic Reserve on Waiheke Island have reopened for tours.
Fort Stony Batter Heritage Park project director and Waiheke resident Timothy Moon says the installation is an important part of New Zealand's history.
Stony Batter was the largest of three gun fortresses built in New Zealand following the 1941 attack by the Japanese on Pearl Harbour. The other two in Whangaparoa and Wellington.
"It's a true fortress, absolutely massive, four times larger than Sky Tower," Moon says.
The fortress is located on the east of Waiheke island in farmland amid a volcanic boulder field.
Stony Batter has 1.2 kilometres of tunnels and has eight large chambers dug deep underground. There are three surface gun pits which were designed in such a way that when the fortress was in operation all gun turrets could be accessed without ever surfacing.
"They are truly vast, it's a massive structure and they were dug by hand with pick axes and shovels and the material was transported out on light gauge tracks and it was completed in two and half years."
The tunnels were reinforced with 60cms of concrete so it could withstand a direct hit.
When the war ended and new technology such as ICB missiles emerged gun emplacements such as Stony Batter became redundant, Moon says.
By 1950 the government started to decommission the complex, stripping out the guns, any steels and secret technology within the site.
"It just remained abandoned for about 55 to 60 years."
The tunnels became flooded, he says.
"There were dead sheep and dead pūkeko in there."
Restoration started 10 years ago, Moon says.
"It's in remarkable condition, it's in almost like new condition and given that it is an 80-year-old concrete building, deep underground, there is no deterioration in the concrete anywhere within the entire complex."
Moon started to talk to the Department of Conservation about making the site a visitor attraction three years ago.
"It is such a magnificent heritage structure that I couldn't see it be left fallow."
Although tricky to access via an unsealed gravel road, visitors are rewarded with a beautiful walk for about 800 metres through the farm and through the boulder fields and past remnant forest to arrive at the tunnels, he says.
Once inside, visitors can explore magazine rooms larger than the average house, gun turrets and go deep underground to a passage that is 400 metres long.
The largest chamber has been turned into a museum and music space, he says.
"The acoustics are insane, they are completely different to what you can achieve in a recording studio and we really attribute that to the length of the passages. You can almost feel the sound move."
So powerful are the acoustics that no amplification is needed, he says.
"In the concert chamber we have a piano, and other musical instruments, and you can be 400 or 500 metres away and hear every single note."
Tours are approximately 1 hour in duration, suitable for all ages and average fitness. The fee for adults is $25, children 12-16 is $15, under 12 is $10 and under 5 free with adult.