Police are investigating all recent cases in which white powder was sent through the mail to see if they are connected.
The move follows the evacuation of 80 people from a central city building about 12.30pm on Monday. The building on The Terrace houses offices for the Treasury and the Ministry of Health and is near Parliament.
Three people were contaminated when the envelope addressed to the Ministry of Health containing white powder was opened in the morning.
Police said although they are still unsure what the powder is, they don't believe that it is harmful.
Senior Sergeant Hamish Milne said the circumstances are very similar to last week where the offices of independent MP Peter Dunne and government minister Gerry Brownlee received envelopes containing white powder. In those cases, the substance turned out to be baking soda.
"Certainly, it is a frustrating thing for us to have to deal with people sending powder in the envelope like this," Mr Milne said.
Five fire appliances, five police cars and an ambulance attended the incident.
Meanwhile, tests have confirmed that white powder in an envelope sent to a Greymouth man is not toxic.
The man took the envelope to police on Sunday after he found that it contained powder and a sinister note. The discovery led to police cordoning off a large part of the normally quiet central business district for a couple of hours.