It marks the highest sale price for a commercial office building in the Canberra area covered by data firm Cityscope.

The transaction sees the two companies take over the site from Mirae Asset Global Investments, a South Korean investment company that reportedly bought the property for $321 million in 2017.

Under the joint venture, GIC will acquire a 95 per cent interest in the property, while Charter Hall will acquire 5 per cent.

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The building, which spans about 40,000 square metres of office space, is wholly tenanted by the federal government's Department of Education, Skills and Employment.

With its lease due to expire in 2025, the department began looking for new office space in 2020.

Kishore Gotety, co-head of real estate at GIC, said Canberra's stable office market made the purchase a strong investment.

"The Canberra market has performed well during COVID, with continued macroeconomic growth and low office vacancy," he said in a statement.

"Going forward, we expect steady office demand, underpinned by healthy employment growth and government demand."

He added the company would leverage Charter Hall's "strong leasing capabilities" to add further value to the investment.

The transaction follows the release of new data from the Property Council of Australia, showing a modest rise in ACT workers returning to the office since the COVID-19 lockdown ended.

The November survey of office owners in the territory found occupancy rates rose from 7 to 17 per cent.

Meanwhile Canberra's office vacancy rate, calculated on whether a lease is in place for an office space, was among the healthiest in the country at 7.7 per cent in August.

Adina Cirson, ACT executive director at the Property Council of Australia, said the $335 million acquisition shows "great strength of confidence" in Canberra's office market.

"50 per cent of our office market is tenanted by the Commonwealth and that gives us a lot of strength when it comes to security of tenure," she said.

"Having someone like Charter Hall investing in Canberra consistently is really going to send a strong message to other investors around the country and overseas.

"So it's really positive and matches up with the confidence that we're starting to see return to the property sector here in Canberra."

Michael Andrews of CBRE, which managed the sale campaign, said investment activity in Canberra's office market was on track to hit a record in 2021.

"[Canberra is] currently tracking $1.17 billion in total office sales, which has completely eclipsed the decade average of $375 million, with another $300 million in sales in the pipeline," he said.

Among this year's major office transactions was the Louisa Lawson Building at 25 Cowlishaw Street in Tuggeranong, which sold for $306 million, CoreLogic data shows.

In September, Lendlease sold 25 Constitution Avenue in Canberra City to Real Asset Management for $115.1 million.

Meanwhile, 2-6 Bowes Street, Phillip was listed for sale in September and 40 Bunda Street in Civic hit the market in October.

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