Spread across the entire 100th floor of what will be the southern hemisphere's tallest building, the ultra luxury penthouse is the most expensive single apartment ever sold in Australia.

The buyer, a businessman based in China - whose identity has not been disclosed - has inked the off-the-plan deal, agency CBRE has confirmed.

Building has started and Australia 108 is due for completion in 2019.

The sale launches Melbourne into the national stratosphere of luxury real estate.

The previous benchmark was set by the sale of two apartments in Sydney's Bondi for a combined $25 million in November last year.

The off-the-plan purchase of the amalgamated apartments atop The Bondi Pacific development was made in a company name linked to Andrew Roberts, the former chief executive of Multiplex.

The previous highest recorded apartment sale in Melbourne was a $19.3 million pad in the deluxe 150 East complex.

On the edge of the Fitzroy Gardens, 150 East is dubbed the "tower of power" for its wealthy and influential residents from business and entertainment.

The sale of the 750 square metre penthouse of Australia 108, which will hover in the clouds at 319 metres above the Southbank promenade, was promoted as the highest dwelling in the southern hemisphere.

The new owner will enter through a private grand foyer on level 98, and have use of a private glass elevator.

He will come home to a multi-million dollar view - the apartment takes in a 360 degree vista of city, the bay and the mountains.

 

Australia 108 from CBRE on Vimeo .

Architecture firm Fender Katsalidis - the team also behind the neighbouring Eureka skyscraper - designed the penthouse.

Over 1000 apartments of the 1105 on offer at Australia 108 have sold since they went on the market in February.

Two types of apartments are available in Australia 108 - the so-called Cloud Residences above the gold "starburst", of which there are 161, and 944 apartments in the portion below.

The penthouse deal was done between CBRE Residential Projects and Tim Kennan, owner of KM Sales and Marketing, who was representing the project's Singapore-based developer World Class Land.

CBRE managing director of residential projects Andrew Leoncelli told Domain last month there were three local buyers who had shown interest in the penthouse.

"The interested parties couldn't be more different with their background, their demand and use of the property," he said.

There had been dozens of inquiries for the penthouse, which had been marketed with a "certain level of sophistication" in the package of printed material they handed out to prospective buyers, Mr Leoncelli said.

"Some of it goes to the highest level of detail and quality and consideration, so it's different to what we give to people on the lower levels," he said.

Australia 108 trumps the height of the 297 metre-tall Eureka tower and has secured the monster sale that has not been achieved at Eureka.

A full-floor apartment in Eureka, occupying all of level 82 - with 360 degree views and a plush, designer fit-out - has been on the market through RT Edgar and Christie's for several months, with an expected price of $20 million.

The 82nd Eureka penthouse, like the Australia 108 penthouse, is designed by architecture firm Fender Katsalidis. It's owned by self-made business and property developer Elias Jreissati.