Veteran playmaker Craig McMahon showed all his years of experience to score one of the sneakiest tries of the season at a crucial time in a qualifying final against the Tuggeranong Vikings.

Everyone on the field was looking the wrong way when McMahon, who came out of retirement this year as a 38-year-old, saw his chance and took it.

It put Royals ahead by one point and became the turning point in a 21-13 triumph at Viking Park on Saturday.

The moment of class and patience was the shining light in an otherwise error-riddled clash, which left Vikings coach Nick Scrivener seething at his team's mistakes.

Royals, however, showed plenty of grit to hold on for victory and advance straight to the grand final.

McMahon led Royals to a drought-breaking premiership four years ago before deciding to retire. Royals coach Wayne Southwell asked him to make a comeback this season and the SOS paid off in the best possible way.

"He's come into his own in the last few weeks. He took a little while to get back into it," Southwell beamed.

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"Is he a cool head? Yep. Because of the respect people respond to him. He knows what to say and when to say it, but he doesn't overdo it."

The Vikings will have to regroup for a preliminary final next week, with their opponent to the winner of an elimination clash between the Gungahlin Eagles and the Uni-Norths Owls on Sunday.

Tuggeranong, the defending premiers and first-placed team in the regular season, started strong against Royals and led 13-7 at half-time.

But they were making errors at an eye-watering rate as they struggled to hold on to the ball long enough to build any pressure.

"There were so many mistakes. We turned it over so easily, I don't know if we got over three phases the entire game," Scrivener said.

"We were poor, especially in the backs. The backs might have to do scrum training this week because the forwards packed about 20 scrums.

"We got what we deserved, Royals were better. It's a good lesson for the blokes to respect the footy. But a lesson is only a good lesson if it's learnt."

Royals took their chance when McMahon spotted a tiny gap and Lincoln Smith put the result beyond doubt when he scored in the same corner with three minutes left.

"To hang in ... it was pure heart. We've flown under the radar a bit and that's fine," Southwell said.

"But that was a fantastic effort. You've got to stay in the fight and then take your chance."

Both teams will count the costs of injuries to key players, but Tuggeranong faces the tough challenge of having to back up next week.

In the elimination final, Gungahlin will be fired-up to make their mark after a last-round hiccup cost them a chance to win their first minor premiership since 2007.

The Owls are aiming for their first John I Dent Cup finals win in almost 20 years.

Meanwhile, the Uni-Norths Owls are through to the women's grand final after beating Easts, but Tuggeranong's hopes of back to back titles are over after they lost to Royals.

AT A GLANCE

John I Dent Cup: ROYALS 21 (Seth Going, Craig McMahon, Lincoln Smith tries; Pedro Roland 3 conversions) bt TUGGERANONG VIKINGS 13 (Len Ikitau try; Noah Lolesio 2 penalties, conversion).

Women: Uni-Norths Owls 47 bt Easts 0, Royals 41 bt Tuggeranong 7

Sunday: Gungahlin Eagles v Uni-Norths Owls at Viking Park, 3.25pm.