The rise of the dark web has led to a huge increase in the amount of illegal drugs being sent from the Netherlands, one of the world’s largest drug producers, via letters and packages in the post.

The Dutch customs authority has warned that a sharp rise in successful seizures from the country’s postal service is indicative of a growing trend that it is struggling to crack.

The amount of ecstasy and amphetamines found in the post more than tripled from 137kg in 2016 to 460kg last year, customs officials said, but those numbers are regarded as a tiny fraction of the bigger picture.

A seizure requires a scanner to detect an irregularity in a letter or parcel and a magistrate to approve a request for a package to be opened.

“We are seeing an increase in all types of synthetic drugs,” said Kim Kuipers, a senior official at the Dutch customs authority. “But by far the most that we see going outside the EU from the Netherlands is ecstasy and amphetamine.”

Criminal organisations in the Netherlands are major producers and exporters of synthetic drugs, exported via sea containers or trucks, but there is said to have been a growth in online purchases in which the postal services are exploited. The US, Australia and New Zealand are the main recipients outside Europe.

The dark web, a series of websites on an encrypted network that cannot be found by using traditional search engines, has been the focus of a series of recent police operations.

Last year agencies in the US, Canada and Europe, including the UK, made 61 arrests and seized nearly 300kg of drugs, 51 firearms and more than €6.2m (£5.4m) after targeting 50 dark web accounts.

But Neeltje Keeris, from the Dutch national public prosecutor’s office, told De Morgen that the problem of drugs being sent in the Dutch post was “only getting bigger”.

“It is difficult to say exactly how big the problem is,” she said. “Those who do this work anonymously. And not all mail can be checked. That is impossible, when you see the huge amounts of mail.”

Last year the Dutch police association warned in a report that the Netherlands was starting to resemble a “narco-state” with forces unable to combat the rise of organised crime and a parallel criminal economy.

Critics of the Netherlands’ gedoogbeleid (tolerance policy) towards the sale of cannabis, and the legal status of prostitution in the country, claim the Netherlands has been promoted as a major drugs hub.

A large majority of ecstasy taken in Europe and the US comes from laboratories in the south of the Netherlands, said to be run increasingly by Moroccan gangs involved in the production of cannabis. Half of the €5.7bn a year of cocaine taken in Europe comes through the port of Rotterdam, according to Europol.

US and Australian customs authorities say that they examine parcels from the Netherlands with extra care. Last year, four men were arrested in Brunssum, Limburg, suspected of drug trafficking on the dark web using the postal service.