Police are issuing a warning after the tragic death of a Canterbury man over the weekend and two other serious incidents in Queenstown.

Police are continuing to investigate the sudden death of the 20-year-old who died in Christchurch in the early hours of Sunday, September 4.

Early indications are his death may relate to a potential drug overdose and enquiries into this are ongoing, says Detective Senior Sergeant Malcolm Inglis.

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

"They have requested privacy at this difficult time."

Police have arrested and charged two people, a male and female, both aged 33, with possession for supply of a Class B drug and we cannot rule out the possibility of more serious charges.

They are expected to appear in the Christchurch District Court today.

Police have been made aware of security guards in Queenstown over the weekend assisting two people who had seizures after reportedly taking MDMA, says Malcolm.

"They didn't require medical treatment, but these incidents could have ended very differently.

"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.

"With a music festival in Queenstown from tomorrow until Sunday, Police are issuing this warning as we don't want to see anyone becoming sick or worse."

If you or someone you know experience concerning or unexpected effects after taking something, please report it to High Alert, New Zealand's drug early warning system, at highalert.org.nz.

This is an anonymous function and helps keep others safe, says Malcolm.