An incident in which the wrong side of a cancer patient's bowel was removed during surgery at Sydney's Northern Beaches Hospital is being investigated.

The NSW government has promised to make sure there is no repeat of the "horrible mistake" that saw the wrong side of a patient's bowel removed at a Sydney hospital.

The error happened at Sydney's troubled new Northern Beaches Hospital last week. It's understood the male cancer patient underwent corrective surgery on Friday.

Issues with the patient's pathology results from a private provider contributed to the mistake in some way, it has been alleged.

NSW Health Minister Brad Hazzard said the hospital had advised NSW Health of the "serious event".

"In such situations there are investigative processes and those must proceed to their conclusion before any further comment can be made," Mr Hazzard said in a statement on Monday.

"The patient's privacy must also be respected."

Mr Hazzard has asked the hospital to provide all possible support for the patient and his family.

A spokesman for the 488-bed hospital - which only opened in October - said the cause of the blunder is under investigation.

"The patient and their family have been provided full disclosure and NBH is supporting their ongoing care and treatment," he said in a statement.

NSW Premier Gladys Berejiklian said she was very concerned to hear about the incident, which she described as a "horrible, horrible mistake".

"We need to find out the root cause of the issue and once that's determined we need to make sure it never happens again," she told reporters on Monday.

"But our first thought is to make sure the patient is given the best care possible under these terrible circumstances."