Inland Revenue hopes to secure short-term office space in central Wellington by the end of September, after staff were sent home because of a seismic risk.
All of the department's 1000 staff based at the Asteron Centre have been working remotely for two weeks.
A fresh detailed seismic assessment of the Featherston St building was commissioned when Inland Revenue was looking to renew its lease there.
It revealed Wellington's largest single office building has a lower New Building Standard (NBS) rating than previously thought.
Staff have been told they cannot be in the building while the seismic issues are being fixed.
Inland Revenue was working with the building owner on a solution, which could take some time, 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, the Inland Revenue spokesperson said.
"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."
Meanwhile, PSA organiser Alan Rattray said he was concerned that union members were not being compensated for working from home.
"Inland Revenue says they are taking a balanced approach, that staff are no longer having to commute, but that's not ideal. Those workers are working from home through no fault of their own."
An Inland Revenue spokesperson said the department did not comment on employment matters.
Rattray said the union supported Inland Revenue's overall decision to move people out of the Asteron building, as health and safety of staff was a top priority.
Inland Revenue has so far refused to say what the Asteron Centre's new NBS rating is, but its guidelines suggest it has dropped by at least 34 per cent.
The Asteron Centre has previously been advertised as being 100 per cent NBS.
Staff must vacate any site identified as being less than 34 per cent NBS, which is the definition of an earthquake-prone building.
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.
Inland Revenue has previously said staff would work from home until a remediation plan was in place to bring the building up to standard.
While these are Inland Revenue's guidelines, under national policy even buildings that have a rating of less than 34 per cent can still be occupied within the timeframe owners have to strengthen them.