Briggs said the council was "crumbling" emotionally after the death.
"Initially we very much said this isn't our fault. The challenge was very much about trying to console the organisation as well. We thought that by saying we didn't do this, it was an unfortunate accident and implied a little bit (that) it was Sam's fault, it would actually help console people.
"The bottom line is that everyone deserves the opportunity to go to work and go home... we failed in that duty regardless of whether or not Sam made a mistake. There was more stuff we should have done, even if she had made a mistake, to make sure she survived.
"People were calling out for us to apologise and take responsibility. We didn't do that straight away. That was a learning I took on board - again the mindset wasn't quite right.
"I'm telling you these things because I'm hoping in some shape and form it convinces you there's a serious part (in risk management) in terms of mindset."
Briggs said he had a "heartwrenching" moment when Samantha's husband Richard, then also employed by the zoo, approached him.
"(He said) why didn't you come to visit me? All I wanted to hear you say was that you cared and you were supporting me."
Briggs said the council's whole mindset about workplace risk and safety had changed since the accident, driven by his personal mindset switch.
"There's a risk that when you write things down (risks) you think you've managed them. It's definitely a mindset that once you identify a risk you think you know it. I don't think I had the mindset to be a chief executive managing the risk in an organisation this large and complex."
The council, which has 26 business units and 1100 staff, brought in consultants PwC to help improve risk management.
Briggs said immediately after the tragedy, the organisation became so fearful of something going wrong, it was at risk of becoming paralysed.
"I was reviewing everything, reading (every report) into the night. I was doing site visits left right and centre.
"That's the wrong mindset because ultimately you are depowering the organisation, upsetting the culture you are trying to build."
Francois Barton, executive director of the Business Leaders' Health and Safety Forum said Briggs' willingness to share this story with more than 80 senior business leaders across the Waikato was extremely powerful.
"His experience outlined a very personal and compelling case for CEOs and boards to grow the right mindset and capability - to care for your people, the senior capability to focus on controlling critical risks and the maturity to enable your people to take ownership and responsibility for safer and healthier work," Barton said.
"Changing our culture of health and safety in NZ needs more leaders to stand up like Richard has today and share their lessons, and their improvement journey."
WorkSafe chief executive Nicole Rosie said change started with leadership.
"Business leaders need be able to be open and honest about learnings from events they are involved in," Rosie said. "This is a key enabler to improving health and safety performance in New Zealand. Others may learn from Richard's real and open perspective on a tragic incident."
