The envelope was opened by Ms Ardern’s staff, just a floor beneath the Prime Minister.
She was due to attend a cabinet meeting on floor ten of the building, after preparing on the floor below.
The mailroom, where the envelope was discovered, is on the eighth floor, and is part of the government’s executive offices, known as the ‘Beehive.’
After an investigation, a police spokesperson reportedly confirmed that the substance was “innocuous,” and “was no risk to anyone involved”.
“The substance is being tested further to identity it,” they added in a statement.
“But we are satisfied that it is not hazardous. It was a small amount, estimated at weighing about 5 grams.”
This scare comes just days after a reported bomb threat in Christchurch airport, which was cleared after the alarm was raised over a suspicious item.
Flights were temporarily halted, and bomb disposal units called while they investigated what initial security reports believed to be an “improved explosive device.”
Several hundred people were affected and all flights were suspended for a brief period, according to the airport.
New Zealand is on high alert after the recent terror attack committed in Auckland, where Ahamed Aathil Mohamed Samsudeen, 32, attacked numerous people in a supermarket.
Samusudeen had been known to law enforcement for a number of years, and was considered one of the country’s most dangerous extremists.
Ms Ardern said: “This was someone who was known to our national security agencies and was of concern and was being monitored constantly.”
READ MORE: Jacinda Ardern wanted New Zealand attacker deported two months ago
Still fresh in the memories of many New Zealanders will be the 2019 terrorist attack in Christchurch, in which 51 people were killed.
Tens more were injured when a white supremacist launched an attack on Christchurch mosques, and live-streamed his actions via social media.
Australian Brenton Tarrant, 30, was given a life sentence without parole in New Zealand’s first conviction for terrorism.
It was the worst terror attack in New Zealand’s history.
Speaking then, Ms Ardern described the terror attack as “extreme and unprecedented violence.”