Agonisingly the winner came in the 83rd minute from substitute defender Khair Jones who had dutifully followed centre-mid Tom Biss along the left flank.
Biss, who needs to put ball distribution ahead of dazzling skills at times, stepped over the ball twice while surging towards the box before flicking the ball to Jones who rifled it with his left foot past a diving WaiBOP goalkeeper Zac Speedy for a 1-0 lead.
"It was a good telling run and a good finish, wasn't it," regular injured captain Finlay Milne said last night following a match televised live on SkySport3.
Asked if Jones would demand a start, Milne said the Manawatu player was carrying "a bit of a knock".
Milne said the Bay United camp was confident that as long as they stuck to their constitution that one elusive chance would materialise in a goal and it duly did.
"We're always working on moving the ball quicker. There's a lot of critics out there but that's how we want o play," said the centreback nursing a hamstring after a short start on Sunday.
Milne said Bay had done well from the short turnaround from Sunday to play away from home.
"We did fine but it was all about our preparation with recovery and managing our training," he said, relieved they won't be playing their next game until Sunday next week.
Striker Samuel Mason-Smith stood out with relentless work, especially the 57th to 59th-minute blitz.
In the 57th, Mason-Smith drove a shot from about 20m out, which Speedy parried innocuously on to the crossbar before regathering as fellow striker Hamish Watson breathing down his neck.
A minute later, Mason-Smith kicked from an oblique angle but Speedy pushed it wide and ditto in the 59th.
Watson nodded a ball over Speedy two minutes later but he should have played the way he was facing, passing the ball back to an advancing defender and two-goal scorer this summer, Kohei Matsumoto, but instead chose to turn with the ball with his back to the goalmouth and, in doing so, blew another scoring opportunity.
A WaiBOP defender brought down Watson in the box in the 62nd minute in what should have been a penalty kick but referee Campbell Kirk-Waugh thought otherwise.
Milne agreed, from where he sat on the bench, it looked like one but felt he needed to revisit TV footage to be convinced.
Three minutes later Watson, with his back to goal mouth at the edge of the 18m box, this time did the right thing in laying a good ball back to a surging Biss who skewed the ball wide.
The injection of Bay English import Stephen Hoyle got WaiBOP on the front foot.
Substitute striker Hoyle put a deft pass across the face of the Bay goalmouth two minutes into added time as Matsumoto and Bay keeper Joshua Hill left it for each other but WaiBOP golden boy Wade Molony, who scored a hattrick in the demolition of Waitakere United the previous round, couldn't poke the ball in from point-blank range at the far post much to the relief of Bay United.
