With Socceroos coach Tony Popovic watching on, Saturday's top-of-the-table clash at McDonald Jones Stadium always promised to be a thriller.
It was Englishman Cosgrove who was the game-defining presence, netting a double to become the first Auckland player to score 10 or more goals in a season.
His brace moved the visitors to within one point of the Jets in the standings and ended their opponents' 10-game unbeaten streak with five rounds remaining.
"I genuinely don't think I've played a game like that in my life. I'm absolutely knackered," Auckland forward Jesse Randall told broadcaster Paramount+ after the match.
"This win is going to give us so much confidence. This is the first time we've beaten them this season."
Newcastle won't just be rueing the loss, with the in-form Clayton Taylor picking up a suspected hamstring injury in the 35th minute.
The injury came just minutes after Taylor had fired the ball from an acute angle into the bottom right corner to level the scores.
Despite the Jets registering about 70 per cent of possession, Auckland were the ones piling on the shots, leading the count 18 to eight.
Newcastle dominated the early chances, but Auckland wrestled back momentum and were rewarded when Cosgrove shuffled the ball into the net in the 15th minute.
The Jets responded when Taylor found the equaliser in the 28th minute from a Daniel Wilmering assist.
Both teams continued to sniff at goal, and in the 52nd minute Cosgrove secured his second when his head found a lofted cross from Francis de Vries.
As full-time beckoned, Newcastle desperately tried to find a moment of magic, but the visitors withstood a barrage of attacks to emerge victorious.
Auckland could have increased the deficit, with Guillermo May's first-half volley clattering into the woodwork, before the Scottish Premiership-bound Randall whacked the ball into the crossbar in the second half.
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