Inside the tunnels of Wellington's WWII fortress

On Anzac Day an underground fortress in Wellington was opened to the public, one of only a few occasions each year that they can get a closer look.

For the most part, Wrights Hill Fortress in Karori was built in secret.

"The workers were sworn to secrecy, they weren't allowed to tell anyone what they were doing" according to tour guide Mike Lee.

The three-gun battery, above Karori, was built in 1942 to protect Wellington from a possible attack by the Japanese.

"The threat was very real because the Japanese had bombed Darwin shortly before", said Mr Lee.

Mr Lee took Newshub through the 620 metres of tunnels constructed in timber, which have concrete poured in behind them.

A camp was built nearby to house 160 workers. 

"They were brought up here by truck, the local people weren't allowed to come in", said Mr Lee.

Inside is a Command Post, gun stores, a diesel engine room to swivel the guns and three "Magazines", which are rooms with 12m shafts that hoisted ammunition up to the guns.

The shells and cordite (gunpowder) charge bags were in separate, triple-bricked rooms to prevent accidental explosions.

Ten men worked in each Magazine. All wore sheepskin "slippers" to reduce the risk of sparks.

The guns arrived from England by ship in 1944.  It was only when they were being trucked up the hill that local people got a glimpse into what was happening on the hilltop.

They could fire up to 30 kilometres - far enough to reach across the Cook Strait towards Tory Channel, or north to Paremata, a suburb of Porirua.

The guns were only ever fired for testing purposes.  Three rounds were fired into the Cook Strait in June 1946.

"They were very accurate, but of course very slow compared to today", said Mr Lee.

Only two of the three guns were delivered, with the war drawing to a close when the third was due.

No guns are at the site today.  They were dismantled in 1960 and sold for scrap, ironically to the Japanese.

The cost of construction of the fortress, in today's terms, was $64m.

The Fortress is open to the public four or five times a year.

The traffic up Wrights Hill Rd was bumper-to-bumper on Anzac Day and queues began to form before it was even due to open.