Postecoglou's side had raced into a commanding half-time lead thanks to strikes from Brennan Johnson and James Maddison.
But Tottenham suffered a second half collapse at the Amex Stadium as Yankuba Minteh, Georginio Rutter and Danny Welbeck scored to hand the visitors their third defeat in seven Premier League games this season.
Postecoglou was fuming after the north Londoners lost a Premier League match they led by two or more goals for just the second time in their last 166 matches.
"Disappointing. Frustrated and absolutely gutted with that. Worst defeat since I've been here. Unacceptable second half," the Australian said.
"Nowhere near where we should be. We got carried away with how we were going.
"We kind of accepted our fate and it is hard to understand as we've not done that while I've been here. We paid the price.
"The problem is we are travelling along too smoothly, football and life will trip you up if you get too far ahead of yourself."
Tottenham had won their previous five games in all competitions, but the manner of their tame second half surrender left Postecoglou raging.
They sit ninth in the Premier League and have two weeks to stew on the devastating blow before returning to action against West Ham.
Asked what he had told his players after the final whistle, the former Celtic boss said: "There is no message. It is a terrible loss for us, as bad as it gets. Only one way to fix it and that's my responsibility.
"We lost all our duels and if you're not competitive it is not going to work. We were not competitive.
"They all go international duty the majority of them. They will process it individually and I will process it when everyone gets back."
Taking his share of blame for the loss, he added: "We've conceded before but it is how you react and our reaction wasn't what it should be.
"It's a bad day for us and when it's a bad day the responsibility falls at my feet."
