Greens leader Adam Bandt has tested negative to COVID-19 and can leave isolation after a staff member of the MP tested positive for the coronavirus on Saturday.
The case, which is not linked to Omicron at this stage, sparked Parliament House's presiding officers to reintroduce COVID-19 safety precautions on Sunday until further notice, based on advice from health officials.
The building was immediately shut to the public on Saturday, the same day it was due to reopen to visitors for the first time after a four-month shutdown.
The Greens staff member was tested in Victoria. ACT Health are still investigating potential exposure locations and close contacts from the final parliamentary sitting week of the year.
It comes as a second Omicron case has been confirmed in the ACT.
In a statement from the Department of Parliament Services, the building's workers were urged to work remotely if possible, including public servants, political staffers and members of the press gallery.
"After discussions with Commonwealth and ACT Health officials, the Presiding Officers have decided temporary and cautionary health and safety measures will be reintroduced to Parliament House from Sunday 5 December until further notice," the statement read.
"Only staff who are considered genuinely and absolutely essential by their employer, and are unable to work remotely, are to come into the building. Where work can be undertaken from home it should be done so."
Parliament wrapped up on Thursday last week, with many political staffers leaving Canberra for the year.
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Visitors to Parliament House will only be signed in for essential meetings, and must re-sign in if they have subsequent meetings in another office.
Face masks will again be mandatory, and non-essential services will also be shut.
The Parliament Shop, which was reopened during the final sitting period for the year, will close again, as will all seated eating areas and the building's Health and Recreational Centre.
These returned restrictions will remain indefinitely as the situation is monitored.
The building was immediately shut to the public after a fully vaccinated staff member of Greens party leader Adam Bandt tested positive to COVID on Saturday.
Close contacts and exposure locations at Parliament House and elsewhere in the ACT between November 30 and December 2 were being investigated.
On Saturday, the Greens issued a statement saying it was a staff member of the party leader.
"A fully vaccinated staff member on Adam Bandt's team, who was in Canberra during the last sitting week, has today [Saturday] tested positive to COVID," the Greens said in a statement.
"They are currently working with ACT Health to determine if they were infectious while in Canberra.
"Other members of the team have been tested and returned negative results.
"Mr Bandt has been tested and is isolating until he receives a result. He is fully vaccinated and not symptomatic."
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