Inside Melbourne Town Hall is an 89-year-old musical instrument that is monumental in both importance and scale.

The grand organ is four storeys high and takes up more floor space than five average homes.

There are two consoles, 552 keys, nearly 10,000 pipes and enough electric wire to run from Melbourne to Adelaide.

It is accessed by a hidden network of metal staircases, corridors and locked doors, and even has its own bathroom and toilet.

Sarah Mary Chadwick is the first organist and songwriter commissioned to create a work specifically for the largest grand romantic organ in the Southern Hemisphere.

While she has studied classical piano and has a small organ at home, Ms Chadwick said she was overwhelmed by the booming sound of the magnificent instrument.

Built in 1929, the Melbourne Town Hall Grand Organ was refurbished and relaunched in 2001. ( Supplied: Melbourne City Council )

Sarah Mary Chadwick says the organ's design is logical and user friendly. ( ABC News: Nicole Mills )

"When I first started [rehearsals] they didn't have this portable console out, so you would sit up on that mezzanine and the whole room had no lights on, it was maybe 10 o'clock at night, there was no-one around — it was literally too scary to play.

"I had it up full and I had to go. It was too terrifying."

While the organ initially appears to be a complex mystery of keys, pedals and stops to be pulled or pushed depending on the sounds required, Ms Chadwick insists the design is logical.

"It's the opposite of digital stuff in that it all kind of speaks to itself and is quite user friendly.

"Instantly you kind of understand. It's so pragmatic."

Unlike a traditional piano, the organ is not touch sensitive so you can't press softly on the keys to generate a quieter sound.

It all comes down to the use of the stops.

Each of the stops on the grand organ can be pulled out to increase the sound. ( ABC News: Nicole Mills )

The biggest challenge is striking a balance between the lyrics and the booming sounds. ( ABC News: Nicole Mills )

"You can't alter how it sounds by how you play it, so if you wanted to play something quiet you might just have two stops pulled out or you might have the harps and the flutes," Ms Chadwick said.

"But if you wanted it to be more abrasive, you might pull out more things."

She said her biggest challenge had been striking a balance between her lyrics and the organ's imposing sound.

"It's pretty crazy. It's definitely super special. I don't know whether I will ever get an opportunity to play something of that scale again.

"It's kind of like if you're allowed to touch everything in the museum."

The original Melbourne Town Hall organ was installed in 1872 and extensively rebuilt in 1905.

It was then destroyed by fire in 1925 and the current organ was installed in 1929.

The Queen Who Stole The Sky is a free performance that will be held at the Melbourne Town Hall on June 15 at 7:30pm.