Auckland FC's unbeaten start to A-League life has come to a crashing halt, after they slumped to a shock 4-0 home defeat to Western United.

United were 3-0 up inside 34 minutes of the match at Go Media Stadium in Auckland, with goals to Riku Danzaki, Noah Botic and Hiroshi Ibusuki.

Matthew Grimaldi put the icing on the cake in the 87th minute, as Western United, who beat Sydney FC 4-3 last week, moved into the top six.

The result marked a remarkable plummet in fortunes for expansion side Auckland, who had won their first six games, before twice coming from behind to draw with Melbourne City 2-2 last week.

The Black Knights had conceded just three goals across 10-and-a-half hours of action during their first seven games, but that stingy defensive record was blown out the water, as United cashed in with three first-half goals in front of 13,681 shellshocked fans.

"It was our home game and our performance was so bad," said Auckland captain Hiroki Sakai. "Four-nil is such a disappointing result.

"I was so embarrassed. For this crowd, we have to give them three points for our next game."

Western United only had 39% possession in the first half, but were by far the dominant side, as they unleashed 13 shots to five.

The visitors were gifted a penalty in the 16th minute, after a handball in the box, and Danzaki made no mistake with the finish.

Auckland goalkeeper Alex Paulsen did well to block Ibusuki's close-range effort in the 23rd minute, but Botic was on hand to put away the rebound with an acrobatic shoulder-height swinging boot.

The home fans squarely blamed referee Casey Reibelt for United's third goal, after the official got in the way of Jake Brimmer, as the midfielder attempted to collect a pass in the centre circle.

With Brimmer unable to get to the ball, Tomoki Imai picked off the pass and raced forward, before finding Ibusuki, who shook off a defender to nail his low strike.

Auckland were lucky not to be 4-0 down in the 43rd minute, with Angus Thurgate's half-volley from the edge of the box cannoning into the upright and bouncing away. Thurgate was brilliant in the first half, with his creativity causing Auckland all sorts of problems.

Auckland coach Steve Corica took off Brimmer, Callan Elliot and Neyder Moreno to introduce Scott Galloway, Jesse Randall and Logan Rogerson for the second half. The moves had an immediate effect, with the hosts creating promising chances to work their way back into the match.

United goalkeeper Matt Sutton pulled off some sharp saves to keep Auckland at bay and Western struck at the other end, when Grimaldi nailed his angled strike in the dying minutes.

The Knights face a tough task to regroup against defending champions Central Coast Mariners at Gosford next Saturday.

Western United 4 (Danzaki penalty, Botic, Ibusuki, Grimaldi) Auckland FC 0