The Cardiff and Vale health board is investigating claims that members of the public were allowed in to watch hospital surgeries.
An internal inquiry has been launched after "deeply concerning" allegations were raised internally by staff during a survey.
But in a statement, the health board admitted that previous concerns about "unauthorised people in theatres" had not been thoroughly investigated or completed.
It's believed the investigation is centring on the University Hospital of Wales in Cardiff.
A CAVUHB spokesperson said: "We want to reassure patients and their families that we are committed to providing safe and high-quality care, and patient safety and confidentiality is always of utmost importance to us."
"We are undertaking a comprehensive and robust review of theatres, particularly with regard to working practices and how teams are structured and managed at the University Hospital of Wales following issues raised by colleagues through an internal staff survey."
The health board said the investigation was already taking slightly longer than the anticipated 12-16 week timeframe, but insisted it was due to "the significant contribution of colleagues".
Theatres procedures are highly sterile and members of the public are not routinely allowed to watch procedures take place.
A spokesperson said there had been occasions where members of the public had been allowed to view operations - including filming for the BBC's "Saving Lives in Cardiff" documentary series and for media packages.
But they added: "These are subject to strict policies and protocols that are adhered to at all times, especially in relation to patient consent."
"This investigation has yet to reach its conclusion but will be conducted and reported to the board as per the health board’s process."
They added the findings would be shared with affected colleagues.
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