Imports of synthetic cannabis have been rising since the Psychoactive Substances Amendment Act took the products off shop shelves last year.
The drug - along with the likes of meth, ecstasy, and Alpha PVP, which is also known as bath salts or flakka - were being found in mailed packages at the country's border.
Customs cargo operations manager Bruce Berry said people try to fly under the radar by going online to order small quantities from overseas.
"What we're seeing is a steady supply of smaller quantities of products in the domain. We're certainly seeing synthetic cannabinoids, we're seeing psychoactive substances, but we're also seeing the mimics there in the same space," he said.
"We realise that a lot of this is people using the anonymity of the web to try to import it for their own personal use."
He said it was part of an international trend.
Wellington Police are cracking down on the issue and have arrested four people after searching six properties in the Hutt Valley and Porirua and finding significant quantities of synthetic cannabis.
The head of the Wellington Organised Crime team, Detective Senior Sergeant Tim Leitch, said more arrests were likely to follow.
"People who are contemplating importing drugs or synthetic cannabis via the internet should really think twice before they do so because it's not anonymous, and there is every chance that the police will come knocking on your door."
He said there had recently been dozens of seizures of the ingredients used to make the drugs, as well as the finished products themselves.
"At one address there were 70 small bags packaged up ready for sale and also, at another address, there was quite a large quantity of tobacco and plant material that either had been or was about to be sprayed with the substance," he said.
"So although the amount across the border might be five grams, by the time it's diluted into a liquid and sprayed onto tobacco it actually goes quite as long way and causes a lot of harm."
Detective Senior Sergeant Leitch said websites which sell the drugs keep cropping up as others are shut down.
Drug Foundation executive director Ross Bell said the constantly changing scene presented a difficulty and a danger.
"For the user, they don't know what they're taking. The health authorities don't know what the health effects are so there's a real risk to public health."
Mr Bell said authorities should publish more information about the types of new drugs being picked up at the border so would-be users think twice before buying them online.