A specialist physician has told the Supreme Court in Brisbane that an infection which developed in a patient who later died had nothing to do with former Bundaberg-based doctor Jayant Patel's surgery.

Patel, 62, has pleaded not guilty to unlawfully killing Mervyn Morris, 75, who died three weeks after a sigmoid colectomy at the Bundaberg Hospital in 2003.

The crown alleges Patel caused Mr Morris' death by performing the operation.

Specialist physician Dawid Smalberger told the court the patient had a one in 10 chance of dying from the surgery, which was performed on Mr Morris to address rectal bleeding.

He said he believed Mr Morris should have been operated on because of his heart condition and concerns about his ability to cope with bleeding without it bringing on a heart attack.

Dr Smalberger said in his opinion, the cause of death was an infection that spread to his blood stream.

But he said it was not related to the surgery, because it developed a week later.

The trial continues.