The Ministry of Social Development says a man went on a rampage at its Christchurch premises, critically injuring himself and harming a Child Youth and Family worker.
The police say the man suffered lacerations and lost a significant amount of blood during the incident on Monday.
He was taken to hospital in a critical condition but is now classified as stable.
Police were called to the ministry's temporary offices in Papanui shortly after 12.30 pm.
They say the man was acting extremely aggressively and was punching and breaking windows and doors.
He reportedly grabbed a female staff member around the neck and shoulder.
Ministry chief executive Brendan Boyle says the events were deeply upsetting and unexpected.
He told Checkpoint the man was challenged by a security guard at the entrance, who had called police when the man did not stop.
The intruder managed to smash his way through a set of secure doors and pushed aside a staff member who confronted him. She is now recovering at home.
Mr Boyle says the man then "just about killed himself" as he began smashing his way around the office, including smashing windows.
Mr Boyle says he has no idea what had caused the man to act as he did although he says he has had contact with Child, Youth and Family.
Police say the man has yet to be arrested but will be spoken to in the next few days.
In the meantime, the ministry has increased security at the office and is reviewing security at all its other workplaces.
Union praises ministry's quick reponse
The union representing social workers - the PSA - has praised the Ministry of Social Development for its quick response to the incident.
A man smashed his way through glass doors at the ministry's offices in the Christchurch suburb of Papanui yesterday, injuring a Child Youth and Family worker.
National Secretary Brenda Pilott says union members who experienced rampage are shocked and shaken.
But she says the ministry responded very quickly to a distressing and difficult situation, jumping in to support people as quickly as possible.