A gun emplacement that was used to defend the New South Wales coastline during World War II has been opened for the first time in three decades, as part of ongoing projects exploring military sites in the state's Port Stephens region.

Charles Tennyson and Mark Rawson are working with local and state government agencies to explore abandoned military sites at Nelson Bay and Mount Tomaree, with the hope of increasing community knowledge about local war history.

Mount Tomaree is the southern head of Port Stephens, and was a key defensive position during the war.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 8 seconds 8 s The view of Port Stephens from Mount Tomaree ( Robert Virtue )

Today the mountain features walking trails that rise to lookouts with spectacular views, and is part of the Tomaree National Park.

But at the height of WWII, the site was crawling with military activity.

Built in 1941, Fort Tomaree, as it was known, housed gun emplacements, search lights, radar facilities and torpedo tubes.

Australian and United States forces were in Port Stephens during the war, and the mountain played an important role as a base for troops to continuously scan the horizon, watching and waiting for a coastal attack from the enemy.

There were two main 152mm gun emplacements on the mountain, which today are derelict and shut off from the world.

On the hunt for tunnels under Mount Tomaree

Mr Tennyson and Mr Rawson are investigating whether a secret US military bunker lies hidden beneath nearby Fly Point at Nelson Bay.

Inside the emplacement were wooden racks, believed to be used for storage during the war. ( Supplied: Charles Tennyson )

When the men heard about the Mount Tomaree gun emplacement, they decided to get permission from the National Parks and Wildlife Service (NPWS) to enter and explore the gun positions.

"We were fairly certain there'd be some sort of underground tunnel system through Tomaree," Mr Rawson said.

"It became relatively obvious that if you've got a gun position with six-inch shells, and if you've got torpedos, you don't normally store them above ground, because that just makes them a target.

"When we enquired about Tomaree, we found out that NPWS knew very little about it.

"They knew it was a gun position, and historically it's important, but they didn't seem to know much else, and they had no maps, no plans, or anything of that nature.

"Given that we were already on site at Nelson Bay, we asked them if they would like us to try and locate the tunnel systems and map them out.

"They were more than happy for that, as that just adds to their information base on the historical site."

Unsealing the doors

The door to gun emplacement one had been welded shut, a decision believed to be due to vandalism incidents in the 1980s.

Mr Tennyson, who was on hand to explore the emplacement, said it had not been opened in decades.

"Those doors have been welded shut for about 30-something years," he said.

"So we were able to get in those, have a bit of a geez inside to see what they had to deal with.

"Like in the Indiana Jones story, it was full of spider webs.

"Basically you had five rooms in total."

A tool room was also found inside the bunker. ( Supplied: Charles Tennyson )

Mr Tennyson said inside was a large tool room, other storage rooms, and a bedroom with eight double bunk beds.

It was built into the hill, was made of thick concrete walls, and had high ceilings.

Mr Tennyson said the largest room measured about 10 metres by 8 metres.

"It was basically for the fellows that were in control of that gun and firing that gun," he said.

"If you can imagine it during the height of a battle, boy they would have been busy.

"You could actually sense the stress and panic that could have been going on at that time.

"Thankfully nothing did happen, but they would have been pretty much on their toes waiting for that day to happen."

Secrets of Mount Tomaree still to be revealed

While the sealed door revealed only a bunker with several rooms, Mr Rawson said the mystery of a tunnel system under the mountain was yet to be solved.

"The gun positions and the torpedo [facilities] were probably entirely separate," he said.

"I suppose they wouldn't want to take the chance of having shells going off with torpedos nearby.

"We figure they were basically separate units that were running the two guns, and another unit was running the torpedo tubes."

Rows of double bunk beds were found in the bunker. ( Supplied: Charles Tennyson )

The gun emplacements have now been resealed, to ensure the site remains safe to the public.

Mr Tennyson said exploring the military sites was important for the Port Stephens community to better understand its past.

"There's a lot of bunkers and concrete and placements all around the hill in itself that you can explore today," Mr Tennyson said.

"Just seeing the magnitude of the extent they went to … to protect the area, it's quite gobsmacking."