Wilson De Courcey scored twice for Newcastle, but the Knights' finals run ended with a 30-14 defeat against the Newtown Jets.
Newtown will meet Penrith in the 2026 The Knock-On Effect NSW Cup Grand Final at CommBank Stadium on Saturday, 26 September.
Newcastle opened with Connor Votano's penalty goal before Jayden Berrell's short pass sent Braden Uele over, giving Newtown a 6-2 lead after Kristian Dixon converted.
Joe Nadenic then collected Riley Pollard's kick, and Dixon's conversion extended the advantage to 12-2.
De Courcey answered before halftime, beating his defender and Taj Ford to score in the corner. Votano converted, reducing the deficit to 12-8.
Newtown seized control after the interval when Sam Stonestreet finished a touchline break and Jaxon Lavender scored from another Pollard kick.
That passage put the Jets 24-8 ahead, although De Courcey's second try revived Newcastle's hopes at 24-14.
The Knights could not break Newtown's defence again, while Dixon's late try secured the victory.
Berrell's dummy-half direction, powerful bench forwards and superior ruck control drove the Jets' performance. Newcastle's 72 per cent completion rate against Penrith the previous week had already been a concern, and further costly errors followed.
Ford remained influential after his two field goals the previous week and helped create Dixon's decisive score.