A 20-year-old Canterbury man has died following a suspected MDMA overdose with police on high-alert ahead of a music festival in Queenstown tomorrow.

Detective Senior Sergeant Malcolm Inglis said there had also been two other serious incidents related to the class B drug in Queenstown last weekend.

NZ Drug Foundation executive director Sarah Helm said fatalities from unadulterated MDMA are exceptionally rare.

Inglis said an investigation was underway following the sudden death of the 20-year-old man who died in Christchurch on Sunday morning.

"Early indications are his death may relate to a potential drug overdose and enquiries into this are ongoing.

"Police would like to issue our deepest sympathies to the young man's family who are devastated by this loss."

The family had requested privacy at this difficult time, Inglis said.

Two people, both aged 33, have been arrested and charged with possession for supply of a Class B drug.

"We cannot rule out the possibility of more serious charges."

The pair, a man and a woman, were expected to appear in the Christchurch District Court tomorrow.

Helm said it was unusual for police to comment on an incident like this without a toxicology report having been completed.

"We don't yet have all of the information, so we'd caution against rushing to conclusions in this case. We don't yet know if the substance this person took was adulterated or not," she said.

"Our thoughts are with this person's friends and whānau."

Inglis said police were also aware of two people in Queenstown who had seizures after reportedly taking the drug.

They were assisted by security guards and did not require medical treatment however "these incidents could have ended very differently", Inglis said.

"The best thing people can do to prevent any risk is not to take illicit drugs.

"Police strongly recommend people take extreme care as what they think they may taking may in fact be a completely different drug or substance."

Inglis said this warning from police came ahead of a music festival in Queenstown tomorrow.

"We don't want to see anyone becoming sick or worse."

Inglis asked anyone who was experienced concerning or unexpected effects after taking something to report it to High Alert, New Zealand's drug early warning system, at highalert.org.nz.

He said the service was anonymous function and helped keep others safe.

Helm said encouraged people to get their drugs checked.

"In the first six months of this year, a third of the drugs we tested were somewhat or completely different to what people thought they were.

"Drug checking is free, legal and completely confidential. It helps people stay safer by telling them what is in their drugs. People can find information about drug checking clinics and tips on how to stay safer at thelevel.org.nz."