It was the visiting side's 11th straight win against the Dunedin-based team, and the 19th in the last 22 meetings.
The win locked up the minor premiership for the Auckland side, with a round to play, and means the defending champion has not lost an ASB Premiership match for 404 days.
On the other side of the ledger, it was Southern United's 10th consecutive loss and all but locked up the wooden spoon for a second straight season.
The home side was content to sit back and give Auckland time on the ball, at times it was like watching the tiki-taka style of football Spanish giant Barcelona enjoys playing.
Southern United hung tough for the majority of the first half, but it was always just going to be a matter of time before the visitors turned a monopoly of possession into goals.
But despite all the possession, Auckland City could only muster one goal in the first half.
It came courtesy of forward Joao Moreira, who sent Southern United goalkeeper Tom Batty the wrong way from the penalty spot in the 38th minute.
Moreira was the man fouled in the penalty area by Southern United captain Seamus Ryder a few moments earlier, a decision Ryder clearly did not agree with.
Auckland City, which boasts players of seven different nationalities, almost made it 2-0 on the halftime whistle, but Batty denied Clayton Lewis with his left hand.
Southern United's only real chance of the first half was 4min from the break, when striker Sam French blazed barely over the crossbar.
The home team's injury woes continued, as coach Mike Fridge was forced into two changes in the first half.
He lost key midfielder Harley Rodeka inside the first 15min to a leg injury, while standout defender Jude Fitzpatrick limped off just before the break.
They were replaced by youth players Lewis Jackson and Ben O'Farrell, respectively.
Auckland City striker Ryan de Vries made it 2-0 just a couple of minutes into the second half, when he got on the end of a long ball and lifted the ball over Batty.
Southern United frustrated Auckland with its plucky defending for much of the half, but Moreira found the back of the net inside the last 10min to complete his brace.
Substitute Andrew Milne completed the rout with his team's fourth goal in the final minute of regulation time.
Southern United will wrap up its season against Canterbury United in Christchurch next weekend.
Southern v Auckland City
The scores
Auckland City 4
Joao Moreira 38, 81; Ryan de Vries 47; Andrew Milne 89
Southern United 0
Halftime: 1-0
