Inland Revenue's Wellington office has been deemed potentially earthquake-prone with the owner of the building given 12 months to prove otherwise.
The government department sent 1000 staff home from the Asteron Centre on Featherston St one month ago after a new seismic assessment found the building had a lower earthquake rating than first thought.
The building has previously been advertised as being 100 per cent NBS and the new rating has not been made public.
The Herald requested a copy of the detailed seismic assessment under the Official Information Act, but Inland Revenue said it did not have one.
However, a statement to tenants on July 16 reveals parts of the building are potentially below 34 per cent of the New Building Standard (NBS).
Anything below this rating is considered earthquake-prone. A building's NBS rating is determined by its weakest part so even if the issue is localised, it still affects the site's overall score.
The detailed seismic assessment was not yet complete at the time the statement was issued, but there was sufficient information indicating some specific elements of the building are potentially below 34 per cent NBS.
Those parts were "very much in the minority" and involved the concrete floor, the statement said.
"We are currently undertaking a refinement process of this assessment which allows us to
better understand more detail around the lower rating and the specific elements involved.
"We note that the main primary structure, consisting of the concrete beams, columns and
walls are rated at above 100 per cent New Building Standard."
More information would be communicated as it came to light including any remediation requirements, the statement said.
Building owner Mark Dunajtschik declined to comment when contacted by the Herald about the Asteron Centre's seismic status.
Wellington City Council confirmed it considered the building to be potentially earthquake-prone and has given the owner 12 months to prove otherwise.
Once the owner has provided more information about the building, the council will then decide whether it is earthquake-prone or not.
Inland Revenue also declined to comment on the July 16 statement.
But in a general update two weeks ago a spokesperson said short-term accommodation was being sought, with the aim of having office space in central Wellington for all staff at the Asteron Centre by the end of September.
"Space is being secured more immediately for some staff who need office space now, and there are also some staff co-locating with colleagues at other government departments.
"This is in addition to the extra accommodation already opened up in Inland Revenue's existing office building in Upper Hutt."
Inland Revenue's guidelines are for staff to vacate any site identified as being less than 34 per cent NBS.
If a site is between 34 and 66 per cent NBS, guidelines advise staff can occupy the building with a plan to remediate the site to a rating above 67 per cent.