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Police carrying out a major bust on a Wellington drug syndicate have uncovered a "lethal combination" of drugs and guns in an Upper Hutt house.

Twenty-five people are now in custody after the year-long undercover operation ended with police storming 25 properties around Wellington this morning.

Operation Hunter involved 150 police - including armed offenders squad members - from around the Wellington district.

Detective Inspector Darrin Thomson said it was particularly concerning to find firearms at a "significant" hydroponics cannabis operation in Upper Hutt.

A man and a woman were arrested at the scene, he said.

"Firearms and drugs are a lethal cocktail, which also present issues for both police and the community."

Thomson said investigators went undercover for 12 months before today's raids.

The 25 people arrested include 22 men and three women, ranging in age from 20 to 58.

They face charges including dealing methamphetamine, LSD, ecstasy, cannabis oil and prescription medicines Ritalin and Valium.

Police carrying out the operation found $100,000 in cash, six vehicles valued at around $150,000 and 200 LSD tabs, with an street value of about $8000.

Mr Thomson said the operation was part of a police effort to break down a major drug syndicate which they believe has sold $2 million of methamphetamine in the last two years.

Many of those arrested had gang connections and their arrest would put a significant dent in Wellington drug dealing, he said.

"I believe we have significantly disrupted the criminal fraternity and drug supply chain across the Wellington District."

The 25 people arrested are appearing in the Masterton, Upper Hutt and Wellington District Courts today.