Adam Reynolds converted three drop goals as South Sydney Rabbitohs claimed a thrilling last-gasp 13-12 NRL semi-finals win over St George Illawarra Dragons.

Souths lost their opening Finals encounter with Melbourne Storm 29-28 but, having been one of four teams to finish level on 34 points at the top of the table, went into the clash with rivals St George as favourites.

Yet the Dragons, who thrashed Brisbane Broncos 48-18 last time out, proved a formidable foe - despite losing second row Tariq Sims to an injury that left him on crutches - in what turned into a battle of the boot between Reynolds and opposition centre Zac Lomax.

Lomax, making just his fourth first grade appearance, edged the Dragons ahead 12-11 from the tee after Reynolds' first drop goal had tilted things in the Rabbitohs' favour.

However, half-back Reynolds was to have the final say and delivered under pressure with two further one-pointers in the final three minutes, setting up a preliminary final with arch rivals Sydney Roosters next weekend.

Reynolds and Lomax traded penalties in the opening 22 minutes before Ben Hunt sliced through the Bunnies' defence to give the Dragons the advantage at the interval.

The boot of Lomax extended St George's lead to eight points but Souths soon came storming back and erased the deficit.

Reynolds converted his own try after combining superbly with Angus Crichton and then knotted things up from the tee.

Souths continued in the ascendancy but St George's stern defence survived three successive sets on their own line.

Hunt then wasted a golden opportunity to put the Dragons back on top as his drop goal skewed wide to the left, and his profligacy was swiftly punished as Reynolds made no mistake with his one-pointer.

But Tyson Frizell stole the ball in the tackle deep in the Rabbitohs' half and the Dragons subsequently earned a penalty that Lomax converted to seemingly put St George on course for an upset.

Yet Reynolds had an answer in the 78th minute after Cameron Murray replicated Frizell to level matters once more and with 17 seconds remaining he produced more heroics to settle an absorbing contest.

With three defenders in his face Reynolds stepped to his right and on the run completed his hat-trick of one-pointers, sparkling jubilant scenes among the Rabbitohs and their supporters as they moved a game away from the Grand Final.