Eutylone is an Amphetamine Type Substance (ATS) and was being sold as ecstasy.
Police identified 20 suspected importations of Eutylone and seized about 400 litres of GBL.
"This has a combined street value of more than $5 million and a Social Harm Index cost of more than $30 million," police said.
Two Wellington men, aged 29 and 35, were arrested as part of the operation and will appear in the Wellington District Court tomorrow.
They were charged with importing a Class B controlled drug, importing a Class C controlled drug, and possession for supply of a class B controlled drug.
Police were also investigating items found at one address that could be used for the manufacture of methamphetamine.
Detective Inspector Darrin Thomson, of Wellington police, said today's raids required months of dedicated investigative work.
"We have been monitoring this syndicate's activity for a number of months, collecting evidence of their alleged drug-dealing activity," he said.
"As a result, today we have deployed investigators and specialist staff across Wellington City to gather further evidence and make arrests for this offending."
"Operation Skipjack will cause significant disruption in the illicit drug market and the seizure of such an enormous of quantity of GBL will certainly reduce and prevent the harm associated with this drug.
"We are sending a clear message to these people that if you are going to derive your income from drug activity that harms our communities, we will be using all the legislation available to disrupt that behaviour."
