To put that in perspective, that's equivalent to three Christ the Redeemer statues, or 17.5 blue whales, or 200 tyrannosaurus rex dinosaurs.

From the outside there are shades of The Hobbit and Bilbo Baggins' cosy hillside home Bag End.

Owners Don and Fiona Firth love it, even if Don has to unconventionally mow the roof twice a year - but he has no gutters to clean out.

From the moment he placed a wiry chair frame on the top of the hill decades ago, gazing across at views of the Valley with an idea, to the now-established home almost hidden yet blended into the natural landscape, the couple haven't looked back.

"I discovered that we could actually move the house site from this not so good spot [at the corner bend of a highway] to this beautiful spot on a ridge here, with two rainforest gullies up the side, beautiful views of the magic Mumbulla mountain, and north facing slope," Don said.

"I sat a chair down on the hillside here and [my wife] came and sat in it one day and [said], 'You know, this is where we should build.'

"Of course that was the intention of the chair," he said with a smile.

The next quest was what to build.

"And I've no idea why we built this other than that I spent my life travelling around Europe and we've spent our lives travelling around the world, and I've seen underground houses in Tunisia and Turkey," he said before laughing. "I haven't been to Cooper Pedy yet."

Don had travelled to architectural and archeological wonders with his family, possibly taking inspiration from a number of historic finds, including a cathedral where graves were systematically and carefully carved underground into rock.

Though in Brogo, he doesn't refer to his property as an underground house, rather an earth-integrated property, which was created by cutting into the side of a hill, building a house, then pouring the dirt back over it.

"The main northside is highly glazed, it's then protected by a series of deciduous trees along the north-east and north-west sides to give winter sun and summer shade, and a gravel courtyard in between those trees and the house to give fire protection," Don said.

"The simplest way to describe the house is as a single-storey, concrete underground carpark supported on 20-odd concrete columns on the front, and two semi-trailer loads of concrete blocks at the back, somewhat rounded shapes at the front, and a parapet around the edge."

Through a door on the side, just past a shrine depicting Bacchus the god of wine and pleasure and Demeter the goddess of the harvest, one enters the void, which is fireproof and waterproof and can hold the participants safely for an hour if a bushfire ravished the countryside.

"Whilst the house is completely fireproof, I'd like the keep the garden," he said.

Within the middle of the property, an exposed and open courtyard allowed light to filter into multiple rooms of the home without it feeling dark, grungy and "underground", while high ceilings made rooms feel grand.

It's a home you would expect to see on Grand Designs, and in fact almost did feature.

However, due to an increase in concrete prices during construction, the deadline to pour the slab before the price rise resulted in the home being "too far along" to warrant an episode.