Chief executive Kevin Burrowes appeared before a parliamentary inquiry into consultancy firms on Thursday, where senators grilled him over PwC's conduct.
The inquiry was set up following revelations partners at PwC passed on confidential Treasury information to boost its private sector business.
Mr Burrowes said the conduct of staff and the actions of PwC should not have occurred and issued a mea culpa.
"What happened is totally unacceptable, for this I am sorry," he told the inquiry.
"We cannot apologise strongly enough for breaching the trust placed in us and we accept the justifiable questions about our trustworthiness and integrity ... we were meant to serve and help you but we have let you down."
Thursday's inquiry is the first time PwC executives have answered questions from parliament about the tax advice scandal since the controversy became public.
Inquiry chair, Liberal senator Richard Colbeck, said the appearance of PwC had been held back in order to allow Australian Federal Police to carry out investigations.
While an internal company report on the conduct of PwC and how its culture should change had been completed, Senator Colbeck said it painted a bleak picture of how it operated.
"It was almost depressing every time I started a new section of the report because it basically reinforced at each level how crap things were inside your business," he said.
"I find it hard in polite terms to describe how offended I am as a member of the then-government that was introducing significant tax changes in the interest of the Australian people."
The report, by former Telstra boss Ziggy Switkowski, found PwC had developed a "whatever it takes attitude" that allowed poor behaviour to be overlooked.
Mr Switkowski was also scathing of an overly collegial culture within the firm, which made it easier for blind spots to be overlooked and for the chief executive's power to be amplified.
While the PwC head apologised for the firm's actions, Mr Burrowes maintained the only breach of Treasury information was the start date of proposed tax laws, and not how companies should be set up.
"To say that the breach of confidential information was used to ... create structures for our clients was incorrect, The breaches of confidential information was the provision of the date, that is all," he said.
"I know taxpayers' money was lost as a consequence of this issue."
Greens senator Barbara Pocock said PwC was trying to hide behind technicalities.
"You hide behind careful language. You say there was no commercial gain, so therefore no problem. To me, that's like saying I went to rob the bank but I was stopped by the coppers, so no problem here," she said.
"You got sprung, they got sprung, that does not make them innocent, and the fact that they did not collect what they thought they could collect does not allow you off the hook."
Mr Burrowes told the inquiry steps should have been in place to avoid the tax advice scandal from taking place.
"I'm not going to sit here and defend what happened," he said.
"It's inappropriate, it should never have happened, they breached their confidentiality agreements ... we should have had processes and controls to catch that, prevent it from happening.
"We apologise to you and we apologise to the people of Australia that we've fallen short of the standards."
The inquiry will also hear from former PwC CEO Luke Sayers about what he knew of the scandal during his tenure as leader between 2012 and 2020.
Financial watchdog ASIC and representatives from Treasury and the Professional Standards Councils will also give evidence.
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