Thirroul captain-coach Nathan Fien kicked a field goal in the second half of extra time to lift the Butchers to an epic 19-18 triumph over Helensburgh in Sunday's major semi-final at WIN Stadium.

Drawing on his 14 years of NRL experience, Fien kept his nerve under pressure, shaving the post with his initial attempt in the first half of extra time before calmly knocking over his second attempt to send the Butchers into the grand final.

The Tigers will have to regroup quickly and shift their focus to this Sunday's final against Wests.

"It was a real grind out there," Fien said.

"Both teams played some pretty good football today, and I'm glad we get the early passage to the grand final. To back up after that will be a big task, but I'm sure the Helensburgh boys will be ready to go. They've been an outstanding side all year and they really pushed us today.

"I can't be prouder of the boys. We did the job throughout the regular season, but nothing's secure until you're in the big show. Fortunately today we were able to get there, even if it was the slimmest of margins."

The Butchers drew first blood after 10 minutes, with fullback Sam Clune pouncing on a deft grubber behind the defensive line.

The Burgh replied with five-eighth Jarrod Boyle's try beside the posts before Thirroul centre Damien Blanch scored in the corner for a 10-6 Butchers lead.

Boyle took over again minutes later, cutting out two players to find an unmarked Azan Turoa on the wing for the Tigers' second try. Eamon Hillen's conversion gave the Burgh their first lead.

That was quickly trumped by Reece Jaegar's penalty goal 40 seconds from half-time and the score stayed 12-12 to the break.

Blanch scored his second try eight minutes into the second half.

The Tigers persevered and were eventually rewarded after the Butchers failed to clear a grubber kick from Boyle.

Winger Wayne Bremner dived from nowhere and cleanly grounded the ball for a Helensburgh try, though Hillen narrowly missed the difficult conversion and the score remained 16-16.

Hillen broke the deadlock with a 61st-minute penalty goal, only for Jaegar to level the score at 18-18 with a penalty goal in the 70th minute.

That's how it stayed until Fien nailed the match-winner.

"We prepared ourselves for that kind of game," the 35-year-old playmaker said.

"Right since preseason training we knew we'd have to be a side that would grind out footy games. We're not the biggest pack in the world and Helensburgh are quite a big side, but credit to the boys for hanging in there and getting the job done. It was a tough encounter and we kept showing up for each other, and that's what this club's all about.

"It's great having the extra week to get ready for the grand final, for the older guys like myself and Aaron Beath and Jamie Feeney. But nothing's done yet. We still have to work hard in the week off and we'll be getting ourselves as prepared as best we can."