Pasifika lost beyond the final siren, 21-18 at HBF Park in Perth.
Western Force were awarded a scrum penalty after the final siren which Bryce Hegarty from eight metres in front off the sticks, scored to give the Western Australia side the victory.
"It's one we let slip," Moana Pasifika coach Aaron Mauger admitted.
"We've got to be better in those moments. We couldn't execute our basics. It's hard to play when you're not catching the ball."
To make it worse for Moana Pasifika, when the score was tied at 18-18 with seven minutes to play the Force had Ollie Callan red carded and three minutes later Siosifa Amone was shown the yellow card.
Rather than wilting with 13 men the Force instead lifted themselves up to secure the win.
"That hurt the boys even though we were good on defence and that is key to how you play, the defence of your team has to be good," said Pasifika defence coach Dale MacLeod.
It began so brightly too when Moana Pasifika took just 28 seconds to touch down on Saturday night when a series of sharp passes allowed Fine Inisi to score.
In the 25th minute Pasifika hooker Samiuela Moli touched down from a rolling maul.
However, prop Sueten Asomua was yellow carded shortly before half-time, and Force hooker Folau Fainga'a scored on the very next play.
Scores were level in the 61st minute when the Force swung it wide after a rolling maul and George Poolman touched down in the corner.
The sideline conversion was missed, and the Force were down to 14 men when Callan was sent off in the 73rd minute for a no-arms tackle that resulted in his shoulder colliding with his opponent's head.
Amone joined him on the side-lines a few minutes later for a high hit, but the Force kept pushing for the win and finally got the result.