The Wellington club has thanked 'Chief' for his work, with the coach confirming his departure after the match at Sky Stadium.
"Giancarlo Italiano has announced his resignation as men's head coach," the message read on Wellington Phoenix's X account.
With tonight's loss, the Phoenix have equalled their worst-ever loss at home.
"To lose 5-0 at home with the season still to play for is unacceptable and I take full responsibility," Italiano said. "I've told the club I tender my resignation.
"It's easier for someone to come in with new energy and refocus the group.
"I think it was clearly evident what went wrong today. We conceded three absolutely sloppy goals, but I think it was more the mentality shift.
"Once we conceded that first one, we got quite rattled very easily and I think that's probably symbolic of the team this season. In big moments, there are big errors that are costing us.
"That could be down to inexperience, that could be down to the younger players, that could come down to the senior not fully focused and that really comes down to me at the end of the day. I do all the training, I provide the environment and, even today, irrespective of Joshy making errors in goal, it was me that picked him."
Phoenix keeper Josh Oluwayemi conceded the opening goal, after drifting outside his area, and being caught out by a long clearing kick that bounced over his head and into the empty net. With his team trailling 4-0, he was subbed off at halftime.
Italiano admitted the talk in the sheds at the break wasn't pleasant.
"There wasn't a discussion," he said, "I went in and basically lost my sh*t, to be fair.
"I told everyone that was unacceptable, and I didn't think they understood the context and gravity of the game. I simply said to them at halftime, 'Right, you're playing for the badge now... all you're playing for is honour'.
"There was no way we could claw back a four-goal lead, no matter how good we are, and I wanted to see something. In fairness, the boys battled in the second half, but the game was already gone."
