The on-loan Tottenham midfielder lit up a nervy and error-strewn tie with a fine strike from outside the penalty area with 20 minutes to go at the County Ground.
But, just like at Griffin Park last weekend, the Bees were awarded a spot-kick with just seconds remaining when Luongo fouled substitute Harry Forrester.
With no Marcello Trotta to steal the ball as he so infamously did against Doncaster last weekend, captain Kevin O'Connor stepped up and thumped a dramatic leveller into the bottom corner to leave the tie perfectly poised for Monday's second leg.
Trotta, whose penalty-taking ability has been the talk of football this week after last weekend's miss cost Brentford automatic promotion, was once again on the bench with Bradley Wright-Phillips partnering Clayton Donaldson up front.
The Bees, who won both previous League One meetings between the sides, were comfortably dealing with a string of Gary Roberts corners in the early exchanges but their hearts were in their mouths when the former Huddersfield winger's deflected long-range effort fizzed over the bar.
Just seconds later Adam Forshaw should have opened the scoring. Slipped through expertly by Wright-Phillips, the former Everton man rolled a low left-footed shot across goal when one on one with Wes Foderingham.
The game suddenly began to roll out of first gear and Adam Rooney was only denied the opener in the 25th minute by a smart stop by Simon Moore after Andy Williams flicked the ball into his path.
Williams could only head straight at Moore from a Roberts cross shortly after before former Swindon captain Jonathan Douglas blazed over at the other end.
Roberts, involved in the play-offs for the fourth successive season, had been the home side's biggest attacking danger in the first but he finished the half with a yellow card for raising his hands to Shaleum Logan.
Both sides were struggling to string passing moves together, with possession surrendered far too easily, but Swindon were beginning to create more in the final third.
Luongo fired straight at Moore, Rooney headed Williams' delivery wide and Alan McCormack's long-ranger flew into the stand before Luongo opened the scoring in the 70th minute.
After being found by Simon Ferry, the Australian looked up and placed a low right-footed shot right into the bottom corner from the edge of the box.
Swindon had their tails up and carved open a great chance for a second four minutes later but, after Rooney ran clear and cut inside, his effort was blocked by the diving figure of captain O'Connor.
At the other end McCormack pulled off a vital backtracking tackle to deny Tom Adeyami inside the box with five minutes remaining.
Four minutes into stoppage time, nifty footwork from the impressive Logan began a move which ended with Harry Forrester being hauled down by Luongo.
And once more Brentford had a last-gasp penalty. O'Connor showed the cool head that Trotta lacked, lashing the ball into Foderingham's bottom-left corner to send the travelling support behind the goal into raptures.