The Ministry for Social Development office in Christchurch's Linwood went into lockdown after a threat on Tuesday.
MSD regional commissioner Dianne McDermott said: "The safety of our clients and staff remains our top priority.
"As this is now a police matter, we're unable to comment further."
A police spokeswoman said they were called to an address on Aldwins Avenue about 8:50am in response to a report of a threat.
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"Early indications are it may be mental health related, however police are on scene now and we are waiting to hear back from local staff," said a spokeswoman.
Police were called to an address on Aldwins Rd at about 8:50am in response to a report of a threat.
"One person is speaking with Police in relation to the incident."
The doors to the office reopened at about 10.35am.
NZME reporter Tim Cronshaw was at the scene, he said the doors were locked and no one was allowed in or out of the building, as MSD customers queued outside.
Cronshaw said a man was taken into police custody and escorted out of the building by officers.