Illawarra Lions player-coach Dave Nolen said his team paid the price for complacency following Saturday's 33-point loss to Holroyd-Parramatta at North Dalton Park.
The Lions had won their previous two Sydney AFL first-division games and headed into round four in third spot, but were no match for a committed Goannas in a 12.10 (82) to 7.7 (49) defeat.
"It was a bit of a shocker," Nolen admitted afterwards.
"A few of the boys thought we only had to go out there and the win would come.
"We got ahead of ourselves and were punished by a determined Parramatta."
With two wins from four rounds, Nolen hopes the first home defeat of the season is a lesson for his young side.
"You have to turn up with the right attitude, and we'll learn from this.
"We've put in some good performances so far this season, but you can't rest on your laurels."
Holroyd-Parramatta booted four goals to one to lead by 19 points at quarter-time, sounding alarm bells for the home side.
The visitors played a solid four quarters.
The Lions threatened only briefly in the second term when they had five scoring shots but managed only the one goal.
Ryan Wells booted two goals. Jake Hogarth, who shifted from the ruck to full forward after half-time, was the Lions' best.
Onballer Daniel Worden and defender Christos Panagakos also played well for the Lions.
Illawarra (two wins, two losses) are back at home on Saturday against Macquarie University (2-2).
First bounce on Saturday is at 2pm.
In last Saturday's round four: Macquarie Uni beat Penrith by 13 points, leaders UTS beat Southern Power by 38 and Uni of NSW beat St George by 48.