Authorities claim to have seized drugs including Flakka, a synthetic stimulant blamed for bizarre crimes overseas including a case of a man having sex with a tree in public.

Wellington Police said a new joint operation with Customs targeted the importers of psychoactive substances and controlled drugs ordered online.

The seized drugs included meth, synthetic cannabinoid, ecstasy, and Alpha PVP, known abroad by names including Flakka.

Police said officers teamed up with Customs staff to raid six addresses in the Hutt Valley and Porirua.

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These raids led to four arrests and a seizure of synthetic cannabinoid.

"Since the increased restrictions of psychoactive substances in 2014 there has been an increase in the unlawful importation of these harmful substances," Senior Sergeant Tim Leitch of the Wellington Organised Crime team said.

The rise of Flakka has sparked moral panic abroad, as the drug in some places reportedly superseded so-called bath salts in popularity.

Bath salts were implicated in obscene and psychotic crimes abroad, including the case of a Florida man who ate another man's face in a cannibal attack.

In another case, a nude man running amok in Indiana and allegedly high on bath salts was tasered in the testicles by police but then deployed ninja-like moves to flee authorities.

Three people in the Wellington region, all of whom currently have name suppression, face dozens of charges of importing Flakka or alpha-Pyrrolidinopentiophenone, a class C controlled substance, after being arrested earlier this year.

Gawker said alpha-PVP had been abused recreationally overseas for more than a decade.

In April, The Associated Press (AP) said alpha-PVP was causing mayhem in Florida.

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"Also known as gravel and readily available for $5 or less a vial, it's a growing problem for police after bursting on the scene in 2013," AP reported.

"It is the latest in a series of synthetic drugs that include Ecstasy and bath salts, but officials say flakka is even easier to obtain in small quantities through the mail. Flakka's active ingredient is a chemical compound called alpha-PVP, which is on the US Drug Enforcement Administration's list of the controlled substances most likely to be abused."

Wellington Police said more arrests were possible and investigations into drug imports would continue.