"As the Operation moved into 'phase 2' this year, police, along with our Customs partners, identified further alleged importations by the syndicate which had changed its methods and was now importing quantities through the mail system," Wellington District Field Crime Manager, Organised Crime, Detective Inspector Darrin Thomson said.
"Due to the diligence of NZ Customs, more than 20 parcels were intercepted and stopped from arriving at the intended location."
More than 7000 litres of GBL are imported in New Zealand from countries that include the Netherlands, United States, Spain and China, with NZ Customs seizing significant amounts of the drug during the investigation.
Police said it would be working with international counterparts on prevention and enforcement.
The New Zealand Drug Harm Index identifies the community harm of these drugs at more than a billion dollars and the street value of the GBL involved is estimated at $28 million.
"Operation Skipjack will cause significant disruption in the illicit drug market and the seizure of such an enormous quantity of GBL will certainly reduce and prevent the harm associated with this drug," Thomson said.
"Accompanied with the 'high alert' drug warning system that was activated in November identifying this issue, Operation Skipjack has significantly impacted this syndicate's activity and the associated harm peddled into our communities."
Over the two phases, a total of 50 charges have been filed.
During the searches, police also seized $60,000 cash and three firearms, including a semi-automatic weapon with 70 rounds of ammunition and a kilo of methamphetamine.