It has invited commercial property owners to express interest in providing offices, saying it would prefer a single building of about 32,000 square metres.

The Australian Taxation Office began looking for options in March, more than two years before leases expire at its Civic offices on Narellan Street and in the Gnabra-Kembery building on Genge Street.

Its search for office space creates a lucrative prospect for the commercial property scene in Canberra. The ATO's 15-year lease at its Genge Street office, expiring in November 2022, is valued at nearly $311m. Austender lists Bayernfonds Kambera GmbH as the supplier.

A 15-year lease for the Narellan Street building, which expires in May 2022, is valued at more than $173m. The building is owned by the Queensland Investment Corporation.

Tender documents prepared by commercial real estate firm Colliers International show the ATO wants an office building in Civic, Barton or Parkes with about 120 secured car spaces.

The Tax Office, one of Canberra's major employers, is proposing a 10-year lease with the option to extend this by two five-year leases.

A Tax Office spokesman confirmed the agency had approached the market because of the looming expiry of leases at its two Canberra sites.

The agency's expressions of interest closed in April, and tender documents show it was due to shortlist them on May 26.

The ATO spokesman said the agency was evaluating expressions of interest, and would not comment on the procurement process.

According to the tender documents, an integrated fitout for an existing building would have to be finished by February 25, 2022 but a new build would need to be completed by October 25, 2021.

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About one in 10 offices in Canberra are unoccupied according to the latest figures from the Property Council, however, the vacancy rate in Civic is higher than the capital's average at 12 per cent. Barton has the lowest vacancy rate at 6.9 per cent.

The Tax Office is preparing for a series of leases to expire across the country soon in Burnie, Geelong, Hobart, Sydney and Parramatta.

It has begun consultations on plans to shut down its Geelong office, affecting 121 staff.

The agency's search for Canberra office space coincides with QIC's unveiling of a smaller scaled back development for its expansion plans on a Cooyong Street car park.

An ATO move from its Narellan Street and Genge Street buildings would further shake up the public service's office footprint in the national capital. The Department of Agriculture, Water and the Environment recently detailed plans to relocate from offices on Marcus Clarke Street and London Circuit in Civic, and Mort Street in Braddon, to a planned new building on the corner of Northbourne Avenue and Cooyong Street.

Correction: A previous version of this story incorrectly stated Queensland Investment Corporation owned the Gnabra-Kembery building on Genge Street.