Deaths caused by sponges left inside patients' bodies after surgery and fatal medication errors have been uncovered in a new review into harmful events at Victorian hospitals.

Some 245 sentinel or "harm events" were uncovered in the 12 months to the end of June 2023, according to a Safer Care Victoria report, up two per cent on the previous year.

Four people died or were seriously harmed due to a foreign object staying in their body after surgery, including surgical sponges or dressings which can lead to infections.

Swabs are counted during procedures and the report said most errors were due to staff changing over during surgery, when a procedure involved two stages or when a dressing was modified.

Surgery or invasive procedures were performed on the wrong side of a patient's body three times while one person underwent the wrong procedure.

Eighteen patients died due to a medication error and eight needed life-saving intervention, with prescribing issues and wrong dosages most commonly to blame.

The report also highlighted several missed cases of testicular torsion, a painful condition commonly affecting young boys and teenagers, which led to serious harm despite each person going to an emergency department with severe abdominal pain.

The total number of harmful events in the year to 2023 was the highest on record since Safer Care Victoria was established in 2017.

Chief executive Louise McKinlay said it was important to learn from every single event so they are not repeated.

"We're seeing a stabilisation in the number of sentinel events being reported to us – this demonstrates an improving culture of transparency on safety risk issues and a willingness to learn from patient harm," she said.

Health minister Mary-Anne Thomas said several changes had been made since the reporting period.

These included a new urgent helpline to ensure patient and family concerns were heard, standardised monitoring across all hospitals and an upcoming 24/7 virtual paediatric consultation system.

"Every sentinel event is a tragedy but it is our responsibility to learn from them and maintain a health system that is safe for every Victorian," Ms Thomas said in a statement.

"We are making significant changes to the way our health services respond to patient deterioration because we know this has historically, and unacceptably, been a significant factor in paediatric sentinel events."

The report demonstrated the pressure the state's health system was under, opposition health spokeswoman Georgie Crozier said.

"Our health system is in crisis, and this report shows that the numbers of people having those shocking incidents occur are not declining," she told reporters in Melbourne.

"They're avoidable incidents and they should be avoided at all costs."