An unexplained New Year's Day death is the subject of a police investigation after several people were found unwell at an address in Hamilton.

Detective Senior Sergeant Andy Saunders said police received a report that multiple people were unwell at an address on Casey Ave around 3.20am yesterday.

One person was found dead at the property and three others taken to Waikato Hospital, one in a serious condition.

Saunders said a scene examination was expected to be completed this morning and a post-mortem examination would also take place today.

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Investigations were continuing and police inquiries would continue, Saunders said.