It was just 1-0 at half-time before the powerful Aucklanders poured on three more goals in a one-sided second half at Fred Taylor Park.
In a New Zealand Football Championship that has developed into a battle between the haves and the have-nots, Waitakere and Auckland City are tied at the top on 23 points.
Otago remains third, on 16 points, followed by Canterbury (14), Wellington (12), Hawkes Bay (11), Manawatu (10) and Waikato (9).
Canterbury has two games in hand, one of which is against Otago in Christchurch and could potentially be a clash that decides third place.
Otago's other remaining games are both at home, against Waikato (Feb 21) and Manawatu (March 13).
United went into yesterday's game feeling justifiably buoyant, being in the rare position of having more wins than losses and coming off a strong 3-1 win against Wellington.
The bubble soon burst as Waitakere scored from a corner after just eight minutes, with former All Whites striker Brent Fisher heading to Jake Butler, who looped in a shot over the head of Otago goalkeeper James Wagget.
Otago's best chance of the half came when Stu Kelly threaded a pass to team-mate Dave Dugdale, who blasted his shot straight at Waitakere keeper Danny Robinson from close range.
Waitakere dominated the opening minutes of the second spell and went 2-0 ahead when Benjamin Totori whipped in a cross, with Fisher again playing provider for Roy Krishna to score.
Two goals in three minutes from Totori and Neil Emblen late in the game sealed Waitakere's comfortable win.
"We were still in the game at half-time but we have to accept we were out-played in the second half," Otago United coach Malcolm Fleming told the Otago Daily Times after the game.
"We didn't keep possession long enough and got what we deserved, I suppose."
Fleming said spirits were still exceptionally high in the Otago squad with three games to play and a rare place in the play-offs beckoning.
"We're third, we're right in the hunt and everyone is cutting each other's throats.
"We don't want to leave our destiny in other people's hands.
"If we hit our straps and get two good results in the next two games, we'll be right."
