Former Bundaberg surgeon Jayant Patel should never have performed the operation that ultimately killed an elderly man in 2003, a court has heard.

The major operation to remove part of Mervyn John Morris's colon was not necessary and shouldn't have been performed on such a frail and elderly patient, the Supreme Court in Brisbane was told on Friday.

Mr Morris, 75, died of complications including heart and lung failure and septicemia, crown prosecutor Peter Davis SC told the court on the first day of Patel's manslaughter trial.

The operation, which was "fairly drastic" and unnecessary, was intended to address bleeding from his rectum, Mr Davis said.

The court was told Mr Morris had been readmitted to the Bundaberg Base Hospital complaining of feeling unwell and bleeding from the rectum after several colon operations under Patel the previous month.

Patel, then the hospital's senior surgeon, performed a sigmoid colectomy and colonoscopy on May 23, 2003, to remove part of his colon and attempt to stop the bleeding.

This was done despite the site of the bleeding being unknown, the court heard.

Evidence from doctors who didn't work at the hospital showed Mr Morris should have been treated conservatively and his condition could have been managed by blood transfusions, Mr Davis said.

The court heard Mr Morris had a history of heart problems, was frail, and there was evidence in his medical notes that he suffered from liver cancer.

"It was ill advised, it didn't benefit Mr Morris, it was a huge strain on him and ought not to have been done," Mr Davis said.

A week later Patel performed emergency surgery on Mr Morris to repair a wound that had been created in his abdominal wall from the earlier operation.

However, he failed to address another complication, which was that the patient's colon was blocked and faeces weren't passing through, Mr Davis said.

From then, medical notes showed Mr Morris steadily declined, with more rectal bleeding, poor appetite and massive problems with his liver function.

On June 14, suffering low blood pressure and low blood sugar, he went into respiratory arrest and died.

Mr Davis said experts would say his death was caused by Patel's operation and Patel's failure to give post-operative care.

Patel has pleased not guilty to the charge of manslaughter.