With 12 minutes remaining, an offside-looking Dack turned in a header from Derrick Williams to draw the hosts level at 2-2.
Assistant referee Rob Smith didn’t raise a flag initially, but was quickly surrounded by Town’s players in protest and then referee Gavin Ward.
After a brief discussion with his assistant, Ward rightly disallowed the goal, as Luton went on to see the game out in the closing stages to emerge triumphant.
Collins said: “I knew he was offside because I was the closest one to him and I was a couple of yards in front of him.
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“I think the decision was to see if he touched it in or not.
“Once the linesman knew that I as quietly confident that he’d give offside.”
Centre half Matty Pearson, whose header just before the hour mark was to prove the winner, said: “For me, it was clearly offside.
“I’m playing and it’s sometimes different, but you’d have to ask the referee
“For one minute I thought he wasn’t going to give it but thankfully he did.
“Once they discussed it, it was that obvious that it was offside.
“I thought it was offside, but sometimes they’re decisions you don’t get.”
Meanwhile, boss Graeme Jones added: “I need to see the goal but we try to play offside. That’s an instruction and it worked.”