A former anaesthetic technician at a Brisbane hospital accused of injecting patients with saline solution instead of painkillers has been granted conditional bail by a magistrate.

Police alleged Evan Leslie Kajewski swapped syringes and stole the drug fentanyl from the Mater Hospital at South Brisbane for personal use.

The 39-year-old appeared in the Brisbane Magistrates Court today on 12 charges, including assault, fraud, stealing and drug offences.

The assault charges relate to two patients who were allegedly injected with saline at the hospital earlier this year.

In the court charge sheet, police opposing bail alleged Mr Kajewski had been in operating theatres on a number of occasions where "persons were having an array of different surgeries".

"Whilst in these theatres, the defendant would take the drug fentanyl from the operating trolley and replace it with a syringe he had prepared containing saline solution," police said.

"It is alleged that the defendant would then take the fentanyl and inject it before returning to work on patients.

"This is alleged to have happened on numerous occasions between November 2016 and May 2017."

Police said the actions of the defendant "put the patients in these surgeries in serious jeopardy".

"They were at times injected with saline solution when expecting pain-killing anaesthetic drug fentanyl," police said.

"Not only this, but due to the defendant's drug use at work, he was then working whilst under the influence of a very strong narcotic agent whilst treating other patients.

"The results of these actions could have been catastrophic and this investigation has shown that it was fortunate that no patients suffered as a result."

Mr Kajewski, who was arrested on Tuesday, no longer works at the hospital.

In a statement, a spokesperson for the hospital said as the matter was under investigation by the police no further comment would be provided.

Mr Kajewski's bail conditions prevent him from attending the Mater Hospital unless seeking medical treatment, and order him to report to police once a week, surrender his passport and not contact witnesses in the case.

The matter will return to court next month.