A sixth, Danny Furlong, joined the ranks in recent weeks and it was he who got the ball rolling against Tasman, slotting home from the spot a few minutes into the second half after fellow Irishman Eric Molloy was brought down.
Southern didn't have to wait long to double their lead through a composed Mulligan finish but Tasman made a game of it with just over 20 minutes remaining when Tinashe Marowa found the net for 2-1. That meant Southern were now facing a nervy finish to the game but, to the relief of O'Reilly, they avoided that fate as Mulligan made sure of the points when following up his own rebounded strike on 72 minutes.
"It's probably the first time we've put 90 minutes together where we were really solid - hard to break down and looked like causing a real threat in the opposition half," O'Reilly said.
"We maybe could've got one or two more but we haven't by any means dominated the whole game, Tasman had their spells as well. It's been tough but we're hoping this is going to kick-start us on to more positive results. It shouldn't be just one win that we can all be delighted about, we're still bottom of the league and that's not where we want to be."
Tasman coach Richard Anderson was hugely disappointed at his side's failure to perform on such a significant day.
"It was a very cagey start, which I think was reflective of where the teams are in the league and how much this game meant," he said. "But then we started the second half so poorly and got what we deserved."
There were also four goals in west Auckland but these were shared evenly between the sides as Singh's late show denied Waitakere a win that would have drawn them level with leaders Team Wellington and Eastern Suburbs, temporarily at least.
The hosts got off to a good start in the scorching sun when Dan Morgan headed home on the quarter-hour mark and it could've got even better for Waitakere when they were awarded a penalty on 22 minutes. Captain Jake Butler tried to place his spot kick into the bottom right corner but didn't get enough on it and goalkeeper Oliver Sail was able to tip the ball onto the post.
Waitakere were presented with the same opportunity to extend their lead late on when Stefan Thelen went one-on-one with Sail and was felled by the young custodian. The responsibility from the spot was handed to Thelen on this occasion and he made no mistake in giving United a 2-0 lead.
With just six minutes of regulation time remaining, Waitakere were odds on to close the game out and secure an important three points but the contest was turned on its head in an action-packed final few minutes. First, Waitakere defender Julyan Collett received his marching orders after a second yellow card on 88 minutes and Singh then gave the Phoenix hope with a well-placed free kick from the edge of the area.
The visitors were given a boost when it was announced four minutes of additional time would be played and they took full advantage, Singh dancing through the Waitakere backline and past goalkeeper Pirmin Strasser before netting from a tight angle to equalise with virtually the last kick of the game.
"It's a really big disappointment because it was a game we totally controlled - it's hard to take," Waitakere coach Chris Milicich said. "They've scrambled a couple of goals at the death and good on them but we have to be more disciplined to close games like that out. When Julyan was sent off that really changed the game and they then had some confidence."
Phoenix counterpart Andy Hedge admitted his team was fortunate to take something from the game.
"Being 2-0 down with five minutes to go, you'd take a draw all day," he said. "Hand on heart, we probably didn't deserve it but football is a funny game."
Hedge also had special words of praise for the efforts of the match-saving Singh.
"He practices those free kicks a lot, he's a worker and wants to be the best at those sorts of things. He's a good little player and can run a game when he wants to, he just has to be a bit more consistent."
The last round of the Stirling Sports Premiership for 2016 will wrap up tomorrow with three more fixtures before the competition takes a break over the festive period and returns on January 7.
Stirling Sports Premiership - Round Nine
Southern United 3 (Danny Furlong pen 48', Andy Mulligan 58', 72')
Tasman United 1 (Tinashe Marowa 68')
HT: 0-0
Waitakere United 2 (Dan Morgan 16', Stefan Thelen pen 83')
Wellington Phoenix U-20 2 (Sarpreet Singh 89', 90' + 4')
HT: 1-0
