Eighteen people have been arrested and charged with drugs and weapons offences following a week-long operation in south Auckland that netted hundreds of thousands of dollars worth of methamphetamine.

Counties Manukau police are lauding their Operation Foiled, which they say achieved "outstanding results" when 50 officers raided 22 properties last week, seizing $41,000 in cash, 311 grams of methamphetamine with a street value of over $300,000, two kilograms of cannabis and 190 LSD tabs.

They also found 420 pills of imitation ecstasy, which are being analysed to find out what exactly is in them.

The six rifles seized included three military-style assault rifles.

Most of the 18 people arrested have since appeared in court, facing either drug-related, money laundering, firearms or stolen vehicle charges.

The Counties Manukau Financial Investigation Unit is looking at six cars, four motorcycles and a house under laws aimed at seizing assets bought through crime proceeds.

Acting crime manager Inspector Gary Hill says recovering 311 grams of methamphetamine in a week is an achievement, and brought the total seized since the end of January to 412 grams.

"The consequences of drugs are harmful, especially methamphetamine. Its impacts are wide-reaching and often translate into other areas of offending such as burglary, stolen cars, serious violence and intimidation."