A sophisticated drug dealing syndicate importing millions of dollars worth of MDMA from the United States has been uncovered in New Zealand.

A New Zealand Police and Customs investigation dubbed Operation Mack has intercepted two identical 330 gram packages of the drug at post office boxes in Wellington and Auckland this week.

The drug had arrived from an address in California, police said.

A 25-year-old Wellington woman had been arrested and was due to appear in Wellington District Court today. She would likely face further charges.

Detective Senior Sergeant Warwick McKee said each package of the drug had a street value of more than $60,000.

There was evidence the syndicate had been dealing in multi-million quantities of the drug and more packages were expected to arrive in the country in the next fortnight. The postage network was on alert and had been given detailed descriptions of what to look for.

The investigation was launched on Tuesday this week after postal staff were ''alerted to some suspicious activity through systems and processes'' with members of the syndicate going to their store.

Security staff were called and customs alerted, McKee said.

He would not reveal the size of the syndicate, but ''they know who they are''. ''Through our investigations we've identified previous importations, but I'm pleased to say we're getting into it and taking this stuff off the streets.

It was unusual for drugs to come from the US - generally it came from the Asia, he said.

The drug would likely be cut with other substances and pressed into pills or put into capsules - destined for the party scene.

A team of 15 police staff in Wellington and another team in Auckland were working on the investigation, which could see some staff working through Christmas.

"Drugs like this do untold damage to our community. Police are pleased to have disrupted the actions of a group whose sole intention was to sell the drugs with the aim of making significant amounts of money.

"The harm that these drugs cause to our community is immense with groups like this targeting the vulnerable.

"Police and Customs have a number of other targets in Auckland and Wellington.

"We are following a number of positive lines of inquiry and further search warrants and interceptions will be executed over the next two weeks."

Police and Customs were also liaising with international authorities.

MDMA - methylenedioxymethamphetamine - is the pure form of the drug Ecstasy.

It is a class B1 drug and the penalty for importing, manufacturing or supplying ecstasy is a maximum of 14 years