He felt they had matched the Aucklanders in the first half but were disappointed they didn't carry on in the second.
"We've not played any where near like what we know we can."
It was poor to concede all three goals, Robertson lamented, believing the second one killed it for the Bay because they felt they were still in it in the second half.
The wave after wave of attacks paid off in the 31st minute with a 1-0 lead when Waitakere captain Jake Butler finished off a Chad Coombes cross at the far post from a repelled cornerkick.
It wasn't until the 50th minute that former Napier City Rovers player Matt Cunneen rose above a gravity-struck Bay defence to put Waitakere up 2-0 from a Coombes cornerkick.
Five minutes later it was 3-0 deservedly to classy midfielder Ryan De Vries, almost a younger version of veteran Fijian international Roy Krishna, after he latched on to a clearance kick from his keeper, Danny Robinson.
Bay goalkeeper Richard Gillespie charged out to deflect the ball in the 18m box after it became obvious De Vries had too much gas for Robertson. The Waitakere striker chested the ball down to his toes and, almost in one motion, toed it into a gaping goalmouth.
While Bay coach Chris Greatholder would have been disappointed, it was a timely reminder for his men on what they have to aspire to if they want to lift the premiership silverware, let alone foot it on the O-League stage.
If anything, having earned a historic play-off berth last Sunday, Bay had the luxury of treating the game as a dress rehearsal without risking life and limb.
It's pointless letting your play-off rivals know every trick up your sleeve in a game with nothing riding on it.
The Bay had their moments. Perhaps the best came in the 18th minute when striker Dakota Lucas got a through ball but his finishing left little to be desired with just Robinson in the way.
The hosts seemed to be sitting back and, for once, the Bay defenders found players not scared to match their physicality.
"That certainly wasn't the game plan. We tried to push high and force our game on them but they're a good side and they have gone all season unbeaten for a good reason," Robertson said, adding Waitakere set the benchmark.
He agreed beating Team Wellington this Sunday in the capital city was vital to play Auckland City in the home-and-away semifinals.
A sickening head clash between Conor Tinnion and Aaron Scott halted the game for a couple of minutes in the 70th minute, with the latter collecting the first yellow card of the game from international referee Peter O'Leary who had a fine spell.
Tinnion had a pretty quiet day, not backing himself to take his trademark pot shots at goal.
However, one can hardly point a finger at the diligent Englishman, who collected a yellow card late in the game, when most of the Bay's attacks came from long balls in the hope of finding striker Dakota Lucas to do the seemingly impossible.
Robertson pointed out the Bay were missing several key players including ex-Waitakere speedy striker Sean Lovemore (out for the season), Jarrod Smith and midfielder Stephen Hindmarch, who often feeds Tinnion go-forward balls but was serving a match suspension.
However, Marshall also emphasised he was missing five players but felt his troops thoroughly deserved to be the winners on the foundation of a "strong, deep squad".
It begged the question: should other teams in the premiership bother turning up to the playoffs with Waitakere's dominance?
"The problem, obviously, with playoffs is that anything can happen.
"They are one-off games and some of it is in the lap of the gods."
Marshall said New Zealand Football needed to revisit the playoff format to make sure the premiership champions should go on to do the business at the higher level (O-League).
"You can finish fourth in this league and still go to the World Cup Champs so I don't know if that's exactly right.
"But, hey, we all knew the rules at the beginning of the season so we'll be working hard to do well in the playoffs as well."
Marshall didn't think there was anything wrong with the playoff format but just that it wasn't a suitable criteria for picking the most consistent representatives for higher honours.
"You are crowned champions in one-off games but it doesn't happen any where else in the world except in Australia, New Zealand, Japan and America."
He said they were the only countries who embraced the format which was the brainchild of codes such as American Football and rugby league.
"We play football [soccer] that the rest of the world play so the league champions should be the league champions.
"Every country in the world plays football and we are among four countries in the whole world who play it like we do with a playoff format to decide the champions, so I don't really understand that."
Marshall said the flight disruption didn't put them off although Bay United kindly deferred the kick-off time to 2.15pm.
The Bay was a beautiful part of New Zealand so Waitakere enjoyed coming down here to play.
