Embedded deep within the inner-city suburbs of Melbourne is a hard-to-access building with an explosive history.

Jack's Magazine is almost impossible to see into and intentionally difficult to find.

There's no direct public vehicle access and only one opening in its bluestone perimeter wall.

Working Heritage is the group charged with looking after redundant heritage buildings on Crown land.

Ross Turnbull, its executive officer, said Jack's Magazine was built in the 1870s as a storage facility for gunpowder that was headed to the Victorian goldfields.

"The site is 12 hectares in size and it's got this amazing bluestone wall around it. It's a bit like Pentridge [prison] really, without the watchtowers or the razor wire," he said.

"It's not ornate but it's a fairly elaborate storage facility."

A rare glimpse inside the vaulted ceiling storage rooms. ( Supplied: Open House Melbourne )

During the 1870s, gunpowder was being imported by the colony of Victoria in large quantities and the customs department needed somewhere secure to store it.

Jack's Magazine was opened in 1878 as a place where those wanting access to gunpowder for commercial use could go to buy it.

"By that time a lot of gold, all the alluvial gold, had been found and they were starting to blow stuff up to get deeper access to get underground gold," Mr Turnbull said.

"You essentially paid the bond or the tax on it and it was released."

Despite its prison-like exterior, Mr Turnbull said he never ceased to be amazed by the beauty of the magazine site.

"It's essentially a warehouse, it's built for storage of explosives, it wasn't a prestige building like the State Library or the Supreme Court," he said.

"But you look at the hand-tooling on the stonework on these magazine buildings and it's beautiful, it's really lovely detailing.

The former storage facility is about 10 kilometres from the CBD. ( Supplied: Open House Melbourne )

"The magazine buildings where the gunpowder was stored are beautiful bluestone-walled buildings with vaulted arched ceilings.

"What that tells you is that in 1870 this city was very, very rich from all the gold and they could spare no expense in building a building like this."

When the magazine was first built it was only accessible from the Maribyrnong River which meanders along its eastern perimeter.

Gunpowder would arrive in Port Philip Bay by ship and then be transported up the river by barge — the best method of transport to ensure that if there was an accidental explosion, it wouldn't do too much damage to the city.

A canal was created to take barges up to a loading dock where the barrels of gunpowder were hauled up, put on trolleys and wheeled along tram tracks leading through the bluestone walls.

Everything was designed to reduce the risk of explosion.

"The trolleys and everything in there was hand-powered," Mr Turnbull said.

"They couldn't use horses because if a horse got scared and bolted or took off with a trolley or two and 50 pounds of gunpowder ... it doesn't bear thinking about.

Those who worked at the facility had to change out of their everyday clothes each morning and put on a pair of woollen overalls and leather moccasins to complete their shift.

"Nothing that could potentially cause an impact, a spark, was allowed anywhere near the gunpowder."

Jack's Magazine was decommissioned in the 1990s. ( Supplied: Jack's Magazine )

These safety concerns also affected the layout of the facility and its relative isolation from the community that has sprung up around it.

While Maribyrnong is now a residential area, it previously played host to mostly industrial land.

"When you build a gunpowder storage facility you don't want people to be able to get to it very easily," Mr Turnbull said.

"It's supposed to be very secure and secret.

"There was no easy direct street access to the place, it was always accessed by the river, and that has meant there's been a whole lot of suburban development that's gone on around it over the last 20 or so years but the actual magazine facility itself has been largely untouched."

Both of the long rectangular bluestone storerooms are surrounded on all sides by a large grassy mound designed to limit the carnage if the worst happened.

"They're there just in case the gunpowder ever went up, the explosion wouldn't transfer to the next gunpowder magazine by shrapnel flying through the air," Mr Turnbull said.

"The idea was the explosion would go upwards rather than outwards."

The bluestone walls of Jack's Magazine are still standing, more than a century after being built. ( Supplied: Open House Melbourne )

Open for visitors

But it wasn't long before gunpowder was superseded by dynamite and TNT.

In the early 1900s Jack's Magazine was transformed into a storage facility for the armaments factory next door and was used right up until the early 1990s when it was decommissioned.

Working Heritage project coordinator Clare Chandler said when the magazine opened, it was called Saltwater River Powder Magazine and has subsequently been known by many names.

"Walter 'Wally' Jack served as foreman and keeper of the magazine from around 1917 until 1943. He saw the site through two world wars and times of immense change," she said.

"Some have said he also had a rather fearsome reputation, so much so that the magazine was seen as his own little kingdom. Whether that's true or not, the name has stuck."

Working Heritage is now in the process of finding a new tenant that could use the precinct for anything from art to education or hospitality.

Mr Turnbull said the site was unusual in that it had remained largely untouched for well over a century.

"There had been some additional buildings built, particularly during World War I, when they obviously needed more storage for war armaments," he said.

"But those original bluestone buildings, the original bluestone wall and the blast mounds are all there today pretty much in exactly the same condition as they were when they were first built."

Jack's Magazine will be open to the public as part of Open House Melbourne, where interesting buildings across the city throw open their doors on July 28 and 29.

Bookings are essential and can be made through the Open House website.

Mr Turnbull said there would be many fantastic places to visit.

"I absolutely encourage everybody to come out for Open House weekend because it's a fantastic way to see some of the hidden gems of our city but also some of the things that aren't hidden at all but you don't normally get access to them," he said.

"They're ...unusual bits of infrastructure that aren't normally open to the public that really explain how this city works."