The sale figure is more than triple the price the developer paid for the block in 2017.

Geocon purchased the former office building from the federal government for $6.25 million, planning to restore the heritage-listed structure and build "the most famous hotel in Australia".

Almost five years later, the vacant building has changed hands again, with new owners Oceana Property Partners securing the property in an off-market sale brokered by Alex McColl of Knight Frank Australia.

The newly-founded investment management firm plans to revitalise West Block and relaunch it as an A-grade commercial office asset with premium end-of-trip facilities, a café and bike storage.

Built in 1926, West Block is located within the Parliamentary Triangle and housed the Prime Minister's department during the Second World War.

At the rear of the 1.7-hectare site stands a small structure known as the 'Bunker' or 'Dugout', where 'Cable Girls' would encrypt and decrypt messages between the Australian Prime Minister and foreign heads of state.

Oceana Property Partners CEO Grant Traub said Canberra's low office vacancy rate coupled with the building's heritage aspects were attractive factors for the firm.

"This building captured my imagination. You just look at it and you appreciate the grandeur of what it was, what it is and what it could become," he said.

"We really appreciate the stability of the occupier market and the quality of the tenant covenants in Canberra and we thought it'd be a great opportunity to regenerate the asset and effectively bring it up to an A-grade commercial office building.

"We really do feel that we are privileged to have this rare opportunity to bring it back to its former glory."

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According to the Property Council of Australia's recent Office Market Report, the ACT's current office vacancy rate is 6.2 per cent, the lowest since 2008.

Geocon managing director Nick Georgalis said it came as no surprise the property had grown in value since 2017.

"In general, the Canberra market has gone through the roof in the last 18 months. In both commercial and residential we are seeing record prices broken each week as investors from bigger markets realise the value found in Canberra," he said.

"West Block is also an attractive building and an important piece of Australian history and culture ... I'm not at all surprised to see the value for Oceana to pay more than $20 million for West Block."

It's unknown whether future hotel developments are still on the cards for Geocon, but more property transactions can be expected from the group.

"Geocon will be active in the market both buying and selling spaces like the style and quality of West Block to continue our pipeline of works," Mr Georgalis said.

Canberra-based commercial designers Capezio Copeland will manage the heritage restoration of the building. Following planning approval, works are expected to be completed by the end of the year.

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