Police have confirmed that controlled drugs were taken from Christchurch Hospital over a period of a month.
Canterbury District Health Board and police are investigating several recent cases of pain medication being diluted or replaced with water at the hospital.
Detective Senior Sergeant David Harvey says the number of controlled drugs which were diluted is not known. The type of drugs involved would not be identified at this stage.
Mr Harvey says police are working with the hospital to look at the processes involved in accessing and dispensing controlled drugs.
The hospital says extra precautions have been put in place to monitor the use of pain relief medication and to ensure that the correct strengths are administered to patients.
Canterbury DHB issued a statement on Tuesday saying it did not know how long the tampering has been going on.
It is possible a healthcare worker with a drug problem may be involved. Addiction is one of the many occupational hazards facing medical staff, according to the Doctors' Health Advisory Service, which helps doctors with addiction problems.
Edwin Whiteside told Morning Report on Wednesday he has only seen a very small number of cases where doctors have used medical supplies from practices or hospitals, and never one where pain medication has been tampered with.
Dr Whiteside believes it is the responsibility of individual DHBs to assess potential problems with staff misusing drugs.