An address in Wellington is being evacuated after police discovered a hand grenade while continuing to search properties following a methamphetamine bust on Tuesday.
Police wouldn't disclose the address.
Six people were arrested earlier on Tuesday for methamphetamine offences after a raid on a number of properties in Porirua.
Police say they've cracked "one of the biggest syndicates in Wellington".
"Having this scale of syndicate taken out of action is huge", said Detective Inspector Mike Arnerich.
A four month investigation targeted the supply of methamphetamine in Wellington by members of the Porirua Mongrel Mob.
Police uncovered around $450,000 in cash, 10 firearms and two stolen motorbikes worth $30,000.
"As a result of our investigation we've been intercepting members of the drug syndicate ... and other information we've gathered through the infiltration of the Mongrel Mob by undercover officers, our case will be that over the last six months they have distributed about 20kg of meth in the Wellington District", Det Insp Mike Arnerich said.
Two of those arrested are senior patched members of the Mongrel Mob.
Between 1-2 kilos of methamphetamine have been seized and it's thought to have been imported.
The police's asset recovery unit has restrained $1.8m worth of assets, including two homes, thirteen vehicles, two boats and a jet-ski.
The two properties seized were in Howick, Auckland, and in Linden, north of Wellington.
A number of safes found at the properties are yet to be examined.
Those arrested in Wellington are a 46-year-old man, a 44-year-old man, a 23-year-old man and a 40-year-old woman.
Most of the drugs were being sold out of the Linden address.
"The address was fortified, which made it difficult for police, reinforced doors, cameras, alarms, and a very steady stream of traffic day and night, 24/7", Det Snr Sgt Tim Leitch said.
"Over the last few weeks there've been hundreds of visits to that address."
"I've been in Wellington in 40 years and we've never had a success with this volume of drugs that have been distributed that I'm aware of", said Det Insp Arnerich.
He wouldn't reveal any details about the undercover operation.
The four people from Wellington were due to appear in Porirua District Court on Tuesday afternoon on charges including supplying a Class A drug, possession to supply and conspiracy to supply.
A man and a woman were also arrested in Auckland as part of the operation and were due to appear in the Manukau District Court on Tuesday afternoon.
Who's buying the methamphetamine?
"Meth cuts across every socio-economic class and in this case it's exactly the same," said said Detection Senior Sergeant Tim Leitch.
"A user may buy a quarter of a gram, if you're buying a gram or more you're most likely selling it."
A gram sells for between $600-$900.
The Ministry of Health's Drug Harm Index estimates the social cost of each kilogram of amphetamine-type drugs to be more than $1.2m.
"The social cost of meth is huge .. every day you can see the cost of what it does to families and the community.
You only have to go to the District Court every day and hear lawyers talk about how their clients have been addicted to methamphetamine", said Det Insp Arnerich.
Police are still searching a number of addresses in relation to the investigation.