St George Illawarra has broken a 31-year premiership drought and clinched its first title as a merged club with a 32-8 NRL grand final victory over a gallant Roosters outfit on Sunday evening.

The Dragons clinched their 16th premiership overall with the club's highest winning score in a grand final, holding the Roosters scoreless after half-time with the league-stingiest defence that helped them to back-to-back minor premierships.

Despite trailing by two points after one half of physically demanding football, the red and whites turned on a near-flawless performance in the second stanza as rain poured down on Sydney's Olympic stadium.

While coach Wayne Bennett was disappointed with his team's showing in the first 40 minutes, he said he was confident the Dragons could bounce back after being down 8-6 at the break.

"I just thought before the game if we got on top of them, we'd get away from them," he told Grandstand.

"These games are so psychological and we've taken them down psychologically and you just lose your way a little bit and that's what happens."

The inclement weather did nothing to deter 82,334 screaming fans, who turned out for the first all-Sydney grand final since 2004 when the Roosters were defeated 16-13 by the Bulldogs at the same venue.

Dragons full-back Darius Boyd was honoured with the Clive Churchill Medal for man of the match in the final, his second premiership after winning with Bennett and the Broncos in 2006.

While both sides were very much guided into the finals by their crafty halves, the decider was a battle of the forwards from the word go as the Roosters' inexperienced pack took it to the vaunted St George Illawarra defence.

"Once we got our confidence in the second half we threw the footy around a bit more, we always knew we could score points, we had nothing to prove," skipper Ben Hornby said.

"Our whole side from 1 to 17 were outstanding, and that's what we get for it, so we're very happy."

The Roosters conceded the first try but shook off some early jitters with two first-half tries to wrestle their way into a half-time lead, but they faltered after the break as silly penalties and handling errors let them down.

Jason Nightingale scored a second-half double down the right flank which all but sealed the result, but Dean Young and Nathan Fien also added four-pointers to the scoreboard as the Dragons surged home.

Historic occasion

The result ensures Bennett's record in season deciders remains perfect at 7-0, while his old enemy Brian Smith is yet to bag his first grand final win from four attempts.

St George Illawarra had contested just one grand final before this one, in its first season under the merged banner in 1999, but in that game the Red V watched Melbourne run down their 14-0 half-time lead to win.

That loss was the fifth consecutive grand final failure for the Dragons, whose last title came in 1979, but they broke the drought in style on Sunday.

Young's father Craig was the last man to captain a winning St George premiership side.

"I think I was meant to do this," Dean Young said.

"I think it was fate."

A win would have been equally meaningful for the Roosters, who were vying to become the first side since Western Suburbs in 1934 to go from the wooden spoon to win the premiership.

Their 2010 successes were thanks in part to redeemed five-eighth Todd Carney, who returned from a 12-month league-imposed ban to clinch the Dally M Medal and help lead the club to its first grand final appearance since 2004.

Carney appeared to suffer a leg injury in a tackle on the Dragons line late in the game but refused to take the medi-cab option and bravely played on despite the lost cause.

Roosters rake Jake Friend, who was sacked by the club last year after a spate of off-field incidents, turned in a monster defensive performance with 60 tackles, and made some solid hit-ups along the way.

Slippery conditions

Rain during the afternoon ensured that errors riddled the early exchanges, with both sides surrendering field position through 13 knock-ons, and the heavens opened in the second half to make things that much more difficult for both sides.

The Dragons got on the board first through a superb piece of kicking from five-eighth Jamie Soward that sent Mark Gasnier over the line.

The points came off the back of a burst up the left sideline from Brett Morris, and on the next play Soward popped a floating chip to the right of the posts and watched as Gasnier reeled in and planted just before the dead ball line.

But Dragons fans could count their lucky stars that the Red V was on the board at all, after replays showed Morris' foot breached the touch line when Kane Linnett tackled him before the scoring play.

Despite looking wayward early on, Smith's men hit back in equally dubious circumstances when Joseph Leilua caught a Mitchell Pearce bomb, but Soward dislodged it with his knee.

Braith Anasta showed brilliant instincts and dived to ground the loose ball in-goal, but there was questions about whether the young Roosters winger had knocked it on in the first place.

Video referee Bill Harrigan ruled that Soward's knee was a legitimate play for the ball, thus it was a one-on-one strip, and the ball was in play for Anasta to score.

Carney was unable to add the extras and level the scores but it did not matter when four minutes later his side took the upper hand.

Soon after, Mitch Aubusson ran an angling line towards the right corner, dummying and juking before sliding over the stripe with a desperate Morris on his back and suddenly the Roosters looked worlds more confident.

Michael Weyman had to be helped off by two trainers looking very groggy after suffering a glancing high hit by Daniel Conn, but was able to return briefly to the fray in the second half.

Conn flew over Weyman very late in a tackle but barely made contact with the St George prop's head, and was placed on report for his troubles.

St George Illawarra: 32 (J Nightingale 2, M Gasnier, D Young, N Fien tries; J Soward 5/5 conversions, B Hornby penalty goal.)

Sydney Roosters: 8 (B Anasta, M Aubusson tries.)