Owner and former broker Peter Ledgerwood bought the penthouse shell for $1.47 million in 2000, shortly after the hospital closed.

Ben Collier of The Agency says that with a penthouse, there's 10/18 Flinton Street.

"It's one of a kind. In my 32 years in real estate, I've never seen anything like it in Sydney," he said.

"The other apartments in this building are fantastic, but they're half the size and don't have the views, scale and volume."

The four-bedroom, four-bathroom and four-balcony apartment totals 581 square meters and has 4-meter-high ceilings, a calacatta marble gas kitchen with a 3-meter-long island and Gaggenau appliances.

"It really is a house in the air. You can feel like you're in a big house, living completely on one level, with a secure five-car garage in the heart of Paddington," said Collier, who put the house up for auction on June 24.

"A landmark in the transformation of the Royal Women's Hospital estate, this full-floor, single-story duplex is a unique residence on 581 square meters," the listing reads.

"With four magnificent timber balconies, the property sets a new benchmark in terms of grandeur." This home in the sky is undoubtedly one of the finest apartments in Sydney.

"The study in proportion and scale, as well as the four-meter high ceilings throughout, create a sense of elegance and the interiors are finished to the most exacting standards."

The Brompton recently sold for $14 million, setting a new price benchmark for Paddington, with the duplex being meticulously transformed into a charming estate.

According to PropTrack, the median home price in Paddington is $2.82 million, down 14.5 percent over the past 12 months.