"We knew they were going to throw everything at us in the second half so we needed to defend and we've done that," Robertson said, after an unconvincing 1-0 win over Wanderers at Park Island in the previous round.
With nothing to lose, he felt Waitakere were always going to throw bodies forward in a gamble to disrupt Bay's game.
"We're delighted with the points that have pushed us to second in the league now."
It wasn't lost on Robertson that if they beat the Dragons in Christchurch next, they would be assured a playoff berth.
"We might be looking at second place so that could potentially be an O-League place. That has been our goal from the start of the season and we're on track to do that."
The centre back said perhaps playing into the stiff wind in the second spell made things harder, although his men had never doubted their ability to be 2-0 up at halftime.
"The benchmark teams have been Waitakere and Auckland and I think we're close enough. I think we're a better team throughout the game today.
"Yes it was tough in the last 10 to 15 minutes but we've been improving all season so we're timing our run right going confident into the playoffs to win the whole competition."
The visitors struggled to find cohesiveness in the first half hour as the Bay dictated terms with some crisp passing and vision.
In the 35th minute, former Bay United defender Ian Hogg collected a yellow card in the 18m box when right-back Ross Haviland clipped his ankle from behind, but all the howling rapidly dissipated when in the next passage of play centre-mid Tom Biss celebrated a 1-0 lead at the other end of the field following a deft feed from Mulligan.
A goal behind golden boot leaders, Biss planted the ball for his sixth one in the bottom left-hand corner with Waitakere goalkeeper Danny Robinson stranded.
In the 44th minute, striker Tomas Mosquera stunned the visitors again with a ranging header from the edge of the 18m box to extend the lead to 2-0 after a curling cross from Haviland from near the halfway mark on the open-air stand side of the sideline.
Keeper Robinson could only scramble from the far post to the near one in desperate bid to keep the ball out.
Bay goalkeeper Richard Gillespie retained his stellar standards, denying striker Allan Pearce a shot from inside the 18m box in the 42nd minute and emulating his feat in the 56th after thwarting a point-blank drive following a pin-ball moment in front of the goal.
Three minutes later, Maxim Manko latched on to a cornerkick, on the heels of another cornerkick from the other side, but Gillespie was equal to the occasion.
Waitakere pulled one back to 2-1 amid deafening silence in the 71st minute when Bay defenders seemed to gift Richard Cardozo all the time in the world to nod in Pearce's curler from near the right-hand corner flag.
In the final four minutes of added time, the pressure took its toll on Pearce, who tripped substitute Jamie Mason after losing possession and copped his share of raspberries from the Bluewater faithful in the main stand.
Goalkeeper Robinson abandoned his posts to encroach in Bay's half to beef up a cornerkick but Pearce and Hogg again lost the script, comically.
Robertson lauded Mosquera's goal, adding he provided impetus up front with his imposing stature.
However, the striker with four goals, who could have been up there with Biss had he not lost two goals against Wanderers SC, looked lethargic in the last 20 minutes and should have been subbed.
Mosquera had a chance to put Waitakere away with a 3-1 lead in the dying minutes with little pressure inside the 18m box following a Biss pass, but his shot skewed off his boot.
Pearce said Waitakere didn't just harbour hopes of a playoff berth but also to win the title.
"In the first 20 minutes our passes were going astray and we were off the pace," he said, as the visitors resorted to kicking and chasing at times.
Pearce said they were "still a force" but, like Auckland City two years ago, were in a transitional phase of rebuilding.
"We're just taking time to bed in," he said, adding finishing fourth was good enough to give any side a sniff of victory.
