Police reopened roads around the Trades Hall in Wellington, after removing a suspicious package on Thursday morning.
A member of the public reported a suspicious bag outside the Trades Hall at around 9 am local time.
The bomb disposal unit blew up the bag later.
The police said it did not contain an explosive device.
Vivian and Marion Streets of Wellington had been cordoned off for hours.
In March 1984, a homemade bomb went off in the foyer of the Trades Hall, killing its caretaker Ernie Abbott. The person who planted the bomb has never been found.