Students heading for the ball at Wellington's Te Papa museum were earlier turned away.
The national museum was evacuated this evening after the suspicious package was found in a loading dock, the police said.
"Police were notified just after 6pm. Te Papa management initiated an evacuation as a precaution.
"Police teams are in attendance assessing the situation."
Te Papa chief executive Geraint Martin told the Herald, "A suitcase was discovered in a part of the museum, in the loading bay. We asked the police to investigate."
He said the army's bomb-disposal experts had been called to deal with the suitcase, which had not been confirmed as a bomb - "we don't know what it is."
"The army have been called and are looking at it, making a decision what to do."
Wellington Girls College was due to hold a school ball at Te Papa tonight but attendees arriving for the event at 7.30pm were being turned away, Stuff reported.
Police were seen guarding the entrance to the building.
It is understood the partygoers were being housed at the nearby Wharewaka on Wellington's waterfront.
Specialist police teams were at the scene investigating.
