The Defence Force blew up a "hazardous" item as police searched a property in West Auckland.
Officers descended on Red Hills Road, Massey, early on Wednesday morning and were seen talking to residents of a house.
They did not leave the scene until 1.30pm.
No-one was seen being arrested, but police were seen speaking to a number of people before escorting them to their vehicles.
The Defence Force said the explosive ordnance disposal team "responded to a request for assistance" from the police.
"An item was identified that appeared to be explosive in nature," a spokesperson said.
"The item was investigated and discovered to be hazardous and was subsequently disposed of by controlled demolition."
The spokesperson did not give any further details about the item.
Earlier, a police spokesperson said officers were conducting a planned search warrant.
"We won't be able to comment further while this is ongoing," police said.
A Red Hills Road resident said he heard a "big explosion" after 10am.
"I'm still shaking, my wife is still shaking," he said.
There had been a large police presence in the area since about 6.30am and officers were still in the area at 11.30am, he said.
"I have no idea what's going on."
A neighbour said police were already at the scene when the "explosion" took place at about 10.10am, and did not seem startled by it.
"It's sort of like they knew it was going to happen," he said.
Another Massey resident said she heard a "massive boom".
She "wasn't sure if it was thunder or a gunshot", she said.
"It reverberated around the whole neighbourhood and I felt the house shake a bit.
"I couldn't see any smoke or smell anything, but around 12pm there a bunch of police and fire trucks zooming through West Auckland."
Morgan Lowe said he saw police at about 10.30am while driving on Red Hills Road.
"I saw cops standing with automatic rifles ... The traffic was slow as there was a large police presence in the area."
An Auckland local who was driving to work early on Wednesday said police were "everywhere".
"I was heading to work at 6.20am and there were heaps and heaps of police. There were also 15 people sitting on the side of the road in their PJs," she said.
Another resident said he heard a loud noise that sounded like a shipping container being dropped about 10.30am.
Last month, police surrounded a property on Red Hills Road in another pre-planned search warrant.
* This story was originally published by Stuff.