A drug that hospitalised 13 people, including a 15-year-old, after a music festival was a potent new drug known as brown sugar, linked to deaths overseas.

Health officials issued a warning in February after nine patients arrived at Christchurch Hospital with symptoms and side effects associated with hard drugs. They were in an agitated state, with dangerously high blood pressure and elevated heart rates.

They believed they had an MDMA-containing substance, commonly known as ecstasy, but police said subsequent medical analysis revealed the substance taken by the patients was actually N-Ethylpentylone.

DEAN KOZANIC/STUFF Detective Inspector Greg Murton said the drug had been linked to deaths overseas.

Often sold as a white or coloured powder, N-Ethylpentylone it can look the same as MDMA, but is stronger and can cause unwitting overdoses when sold as MDMA or ecstasy. The Class C drug emerged in New Zealand last year.

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Canterbury field crime manager Detective Inspector Greg Murton earlier said the drugs were most likely supplied at Electric Avenue, held in Hagley Park on February 24, with about 13,000 people attending.

Nine people, including a 15-year-old, were admitted over that weekend, and another four the following Monday.

Murton said on Wednesday said if the drug was mistaken for MDMA the user would take three times the "prescribed" dosage.

He said people had died overseas from accidental N-Ethylpentylone overdoses.

"The importers, manufacturers and dealers of these types of drugs, in the form of MDMA, synthetics and so called 'party pills' have no scruples about what they put into them, most of which are simply chemicals in varying doses.

"Dealers have no idea of the potency of the drugs they are supplying, nor what is contained within them, or simply do not care," Murton said.

Murton said users were unable to tell what was in any pill or synthetic drug they took.

"[They] are putting themselves at risk of serious harm or death by buying and ingesting 'party pills', MDMA/ecstasy or any kind of synthetic drug."

"That's the likelihood, but can't say that they weren't supplied elsewhere and people went in."

Five Wellingtonians were arrested last week and hundreds of thousands of dollars in cash seized, along with drugs and cars, after an investigation into the importation and sale of N-Ethylpentylone.