McKenzie, who was under fire in the wake of the Kurtley Beale texting scandal and a loss to Argentina, dropped the bombshell at the end of his post-match media conference on Saturday night.
He notified the Australian Rugby Union (ARU) of his decision, regardless of the Bledisloe result, at 10am and only revealed it to his players in the dressing room after the gutting loss.
"I resigned this morning at 10 o'clock," McKenzie said. "I've been a very proud contributor to Australian rugby, but I feel at this point ... there's a bunch of reasons. The intention was win, lose or draw I was going to announce that resignation tonight.
"I didn't actually advise the team or any of the members of staff, so I've just done that," McKenzie said. "I'm not going to go into the detail - you guys can work that out.
"The easiest way for me is to exit stage left and I'll leave you guys to ponder, speculate and I'll write a chapter in my book and then you'll know all about it."
Image 1 of 7: All Black players sing the national anthem prior to kick off. Photo / Getty Images
McKenzie had coaching success at provincial level, helping New South Wales to the Super Rugby final in 2008 - despite being told earlier in the season that his contract wouldn't be renewed - and, after a stint in Europe, returned to Australia and helped lift the Queensland Reds out of the doldrums to a Super Rugby title in 2011.
The World Cup-winning prop was appointed Wallabies coach last August, only days after Robbie Deans - the first foreigner hired as head coach of Australia's top team - was fired.
McKenzie guided the Wallabies in 22 tests for 11 wins, a draw and 10 losses - that span including a stretch of seven consecutive wins.
He received the public backing of the Australian Rugby Union earlier in the week as the squad was dealing with a host of off-field issues, including the disciplinary investigation into Beale and the sudden resignation of the Wallabies business manager Di Patston.
Australian Rugby Union chief executive Bill Pulver blamed media critics for McKenzie's sudden departure.
"I think Australia has lost a great coach and a wonderful bloke and it's very disappointing we have lost a man like him," said Pulver, who attempted to talk him out of the stunning call.
"In essence, Ewen said he was going to struggle to retain the level of support he needs from the playing group and, in my view, because of the character assassination he's suffered in the last two weeks.
"The attack on Ewen was relentless and essentially left him with the view it set him too far back to be an effective rugby coach."
The ARU hopes to appoint a coach before the squad leaves for Europe on Friday.
