Mr Stewart, the former head of investment company AXA NZ, became chief executive of ACC only in September but left yesterday morning after just nine months in the job.
His departure comes hard on the heels of ACC chairman John Judge's resignation on Tuesday and confirmation that directors Rob Campbell and John McCliskie have been told they will not be reappointed.
"I am deeply frustrated that the benefits of the scheme and the great work we do has been overshadowed by recent events," Mr Stewart said.
Those events included the police decision last week not to lay charges against Bronwyn Pullar and TV3's 60 Minutes on Sunday which implied it was ACC managers who first suggested a deal with her for the return of leaked data about thousands of clients.
ACC Minister Judith Collins said Mr Stewart told her on Tuesday "he felt it was time for him to move on".
She said she felt sorry for Mr Stewart. "He's had a tough time."
Greens ACC spokesman Kevin Hague said Mr Stewart's resignation cleared the way for Mrs Collins to lead the process for refreshing ACC by steering it away from what he and others claim has been a focus on denying claims to save money and bringing it back to the principles on which it was founded.
"We need to reverse this culture of disentitlement that's taken hold since 2009 and with those key players - [former ACC minister] Nick Smith, John Judge and Ralph Stewart - gone we've got the environment to do that."
Mr Hague said there were serious questions Mrs Collins needed to answer about her role in the Bronwyn Pullar affair.
But Labour's ACC spokesman, Andrew Little, said Mrs Collins should be the next to go as Mr Stewart's resignation confirmed the depth of the crisis.
"It is an absolute disgrace, and it is entirely the responsibility of that Government", he told the House, and accused Mrs Collins and Dr Smith as her predecessor of driving ACC "into the ground".
Mr Little said ACC now needed a minister who was focused on the needs of ACC claimants rather than on the Government's "tawdry, nasty, filthy little strategy of trying to fleece people and get people to lose their entitlements".
Mrs Collins said it was time for a culture change at ACC but her concerns were about treatment of claimants and their privacy.
"I want to have a situation where ACC has a culture whereby its staff look at their claimants and they ask themselves, 'How would I like to be treated? How would I like my information to be treated?', or, even more so, 'How would I like my mother's information to be treated?"'
Meanwhile, former ACC minister Nick Smith has broken his silence on ACC whistle-blower Bronwyn Pullar.
Dr Smith was forced to resign from Cabinet earlier this year after it was revealed he offered Ms Pullar his support in her long-running fight with the corporation.
Dr Smith told Newstalk ZB opposition and media views on Ms Pullar had been contradictory - while two months ago people were saying she was a villain, she was now being portrayed as a hero.
"And either of those things are true. Bronwyn is a sad case of a very capable person who's had an accident and actually well illustrates the dilemma for many New Zealanders involved in ACC as to what is the appropriate time of rehabilitation," he said.
