The 27-storey building in Geocon's High Society development, known as Tower B, reached the milestone last week but it was not the only record broken.

A 100-metre building in the same development, known as Tower A, became the second tallest after it topped out at the same time.

The buildings are the first high-rises to be more than 100 metres in the nation's capital. To mark the occasion, there are temporary ceremonial trees on the buildings.

Previously the tallest high-rose was the Lovett Tower at 93 metres in the Woden Town Centre.

There are taller structures in Canberra, including Black Mountain at 195 metres and the Gungahlin MF, a guyed mast between Kaleen and Mitchell, at 193 metres.

However, Tower B at High Society is taller than Parliament House, which is 107 metres.

Both buildings at High Society are 27 storeys. Geocon's previous Belconnen development, Wayfarer, was also 27 storeys. Senior project manager Matt Lawson said the reason Tower B of High Society was taller was due to the floor-to-ceiling heights.

"The floor-to-floor heights here are slightly higher so we have 2.9 metres up until level 23 and then from level 23 to the roof it's 3.4 metres," he said.

The development was originally slated to have a public observation deck but this has been scrapped to make way for penthouses.

READ MORE:

Mr Lawson said more than 41,000 cubic metres of concrete had gone into the development, as well as 3400 tonnes of reinforcement and 102,000 square metres of form work.

"The height was something to contend with, getting materials up so high is probably something we have learnt a lot of lessons on," Mr Lawson said.

"But also it's about repetition and doing the same thing over and over again, trying to get something on every level is really important so that's probably our biggest single driver."

The buildings were given development approval in early 2018 and excavation works completed later that year.

High Society is stage two of Geocon's $1 billion mixed-use Republic precinct. It includes 542 apartments and a five-storey hotel.

The broader precinct would include 1300 apartments and about 3000 residents, Geocon operations manager Damian Rumball said. The first residents recently moved into the already-completed stage one of the precinct.

It was expected construction of the project would be over 10 years but Geocon expedited the process after it secured a financing deal with Goldman Sachs in June 2018.

"We're very proud of what we have achieved [and] we can't wait to have it all completed," Mr Rumball said.

High Society is expected to complete construction in September and residents could be in by the end of Christmas, Mr Rumball said.