A Kurt Gidley penalty on the stroke of halftime had given Newcastle an 8-6 lead at the interval.
Gidley, returning from a foot injury that's kept him out for two months, scored from close range after Ben Pomeroy was placed on report for catching the Knights skipper across the neck with his forearm as he was tackled.
The incident led to a minor melee between players from both sides in an entertaining opening period between two sides desperately trying to keep their finals hopes alive.
The Sharks went ahead through Fifita with 10 minutes on the clock after Gagai coughed up possession to Sosaia Feki 30 metres from the Knights' line.
But he atoned for that error just before the half-hour mark by applying the finishing touches to an exquisite offload from Akuila Uate who hauled Mullen kick.
The second half saw the Sharks regain the lead through Wade Graham after the back-rower reacted quickest to get the final touch to Carney's probing kick.
But Tyrone Roberts hit back almost immediately for the visitors after a bellringer of a tackle from Neville Costigan knocked the ball from Nathan Stapleton's grasp.
Prop Kade Snowden picked up the loose ball and sent the halfback over.
The Sharks the proceeded to pin the visitors back in their own half for a 10-minute spell but failed to find a way over and Gagai stunned the home crowd at the death to steal the two points.
Bennett admitted his side got out of jail but said the determination in defence meant they deserved the two points.
"We did everything wrong except give up," Bennett said.
"It was a tough night for us, we're making a lot of mistakes and poor errors.
"But we defended outstanding ... you can't be pretty sometimes."
Sharks coach Stuart Flanagan was fuming at the way his side switched off and allowed Gagai to seal the win.
"We gifted them two tries, one off our try-line, a drop ball into the hands and they score," he said.
"They just wanted that last ball more than we did (at the end). We just needed to defend one more play to get two competition points and that hurts.
"We need a kick up the backside for it."
- AAP