The hospital isn't named in the decision based on an investigation involving seven Waikato DHB staff; consultant, nurses, cardiologist and general surgeon.
The man, aged in his late 60s, had elective surgery in 2016 to remove a 60cm polyp after having a five-year history of bowel problems.
However, following surgery he developed an ileus - when the intestines stop moving food around.
It wasn't picked up and he died five days later of acute heart failure and sepsis.
Dr Kenneth Menzies, who provided external, independent clinical advice of the case, said the man should have been transferred to a different hospital on day four given his symptoms.
The Waikato DHB carried its own serious incident review and found eight failures including not contacting a more senior doctor regarding more pain relief, the imbalance of fluid input and out that wasn't addressed properly, staff overlooking a 1650ml vomit and his family not being notified of his deterioration and when they were, were told all was well.
The commissioner wrote that it was of "significant concern" that hospital staff may have not taken warning signs seriously enough as he was regarded as a low-risk patient who was expected to be trouble-free post operation.
"This perception may have contributed to the failure of staff to perform appropriate assessment and treatment at critical moments in [patient's] care, particularly following his deterioration.
"It is important that staff evaluate patient signs objectively ... and respond accordingly, and be able to adjust treatment plans should a patient deteriorate unexpectedly."
The commissioner was also concerned that the major bowel surgery took place at the hospital at all, as it was only intended for low-risk surgical procedures.
"Major elective surgical procedures should take place only when there are adequate resources and appropriate senior clinicians available to deal with any potential postoperative complications."
As a whole, the commissioner found the failures showed a "pattern of poor care by multiple staff, with the result that Mr B's deteriorating condition was not identified and responded to in an appropriate manner".
He noted that the DHB had since implemented a number of changes since the man's death.