About 19,000 cannabis seeds have been stopped from entering New Zealand this year (file photo).

Cannabis seeds are flooding New Zealand's mail system with 19,000 caught at the border this year alone.

One recent case, involving multiple packages containing more than 30 seeds each, resulted in a significant drug bust in the South Waikato with two men arrested.

The Class C drug is mainly being imported from the Netherlands and United Kingdom, Northern Ports NZ customs manager Dominic Adams said.

GRANT MATTHEW/STUFF Most of the seeds are coming from the Netherlands and the UK (file photo).

"We are finding cannabis seeds regularly in the mail system. In 2016 we got roughly 18,000 seeds; in 2017, 43,000 and it's dropped down to about 19,000 so far this year.

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"These guys will import some seeds, we will seize them but before we have even had a chance to pass the information to police they will order some more," he said.

The illicit mail isn't addressed to hot spots, per se. It covers most of the country, Adams said.

"That is the reason why we work very very closely with the NZ Police, particularly in the districts.

"We see large fluctuations each year on seizure amounts so I wouldn't say it is less or more prevalent now compared to a number of previous years.

"Last year was an anomaly at 43,000, that was quite large," he said.

Senior Sergeant Jason Henderson said two Tokoroa locals were arrested recently following the execution of a search warrant at a house seeds were addressed to.

"At the house we found other drug related equipment and items of interest and just on 100 cannabis plants," he said.

Henderson said people try to import cannabis seeds to guarantee getting female plants.

"The female plants are the ones that grow the buds which hold the highest tetrahydrocannabinol (THC) but if you have a male plant anywhere near them, instead of growing a bud, they will go straight to seed.

"If you get seeds here in NZ chances are you might get a male which doesn't grow any buds so they are basically worth nothing and that is why they are importing them, they are guaranteed females," he said.

Adams said it was not worth the risk with New Zealand Customs and police coming down hard on importers.

"My message is that it is best that you don't because either New Zealand Customs or the NZ Police will come knocking on your door and we will be asking questions as to why.

"The ramifications can be quite serious," he said.

The maximum penalty for importing, growing, cultivating and/or supplying cannabis is eight years imprisonment.