In a blow to its hopes of a top-six finish, Southern was well beaten 4-2 by sixth-placed Tasman United at Sunnyvale Park on Sunday.
The loss means it remains in eighth, widening the gap between itself and the top six to five points with six games remaining.
Two goals just before the break proved too much to overcome.
Southern had looked good early and took the lead via a Stephen Last header from a corner.
However, Tasman hit back in the 38th minute, as tall defender Daniel Allan was left unmarked in the box and made no mistake finishing.
Six minutes, later the key moment came.
Maksym Kowal rocketed a shot which deflected off Last’s lower arm in the box.
A penalty was awarded, although it may have been unlucky.
Last’s arm had not moved and it did not seem to be intentional — as the law states it has to be.
That came off the back of Southern having a free kick turned down, as new signing Alex Ridsdale went down in possession on halfway.
None of that mattered though, with the scoreboard showing 2-1 after Paul Ifill calmly slotted the penalty.
From there, Southern deteriorated.
It became disjointed, its passes going astray, forcing it to defend far more than it had in the first half and it rarely it threatened to score.
Kowal lobbed the ball over goalkeeper Liam Little for Tasman’s third goal.
He found himself one on one with Little — who was forced to come out to it — after a brilliant ball over the top from Ifill.
He added another late in the game when the linesman deemed the ball had crossed the line after a scramble at the goal mouth.
Southern fought back and Ridsdale managed a consolation goal in added time.
It was a result with which Southern coach Paul O’Reilly was disappointed, but he conceded his team did not play well enough to win.
"Bitterly disappointing," he said when asked what he made of the game.
"It was a big game for us and if we had have got a result we would have put ourselves in with a shout of making progress up the table.
"But it was 1-0 after 30-odd minutes.
"I thought we looked really strong, we were dominating.
"I just felt like it was a really strange decision for the penalty, certainly the free kick, not getting the free kick and then the penalty for what it was.
"Then we’ve given away a sloppy one from a set piece.
"Just a bit sloppy. They didn’t have much possession or dominate, but we just turned the ball over too often and [it was] not quite good enough."
He said the penalty was a blow, although did not want to use it as an excuse.
After a very good first 30 minutes, he felt the team did not play well enough from that point on, particularly in the second half.
He said the absence of injured strikers Garbhan Coughlan and Danny Furlong had hurt it, as it did not have another genuine centre forward.
National football league
The scores
Tasman United 4
Daniel Allan 38’, Paul Ifill 44’, Maksym Kowal 65’ 83’.
Southern United 2
Stephen Last 24’, Alex Ridsdale 90’.
Halftime: 2-1 Tasman.
