IT'S a comeback few people could have predicted.

But the Mitchell Magpies have pulled off an improbable victory in their preliminary final on Sunday, overpowering the Chinchilla Bulldogs in an incredible second-half display.

Trailing by 12 points at half-time and eight with just 17 to go, playmakers Tom Kennedy, Griffin Newton and Ben Richards orchestrated a 42-34 boilover to send the Doggies packing from the competition.

With only two players on the bench, few would have given Mitchell a chance against the formidable Chinchilla that beat the previously undefeated Roma Cities earlier in the season.

But against all odds, the Magpies scored four tries in 10 minutes, including a 90-metre intercept try to Newton, to book a re-match with the Gladiators this weekend.

Kennedy, who put on an attacking masterclass at five-eighth, said he reminded his troops about the pride of the jersey.

"At half-time, we just remembered what we wear on our chest (the Magpie) - it's what you play for,” he said.

In the other games, Roma Cities' A-grade and reserve grade sides earned their passage through to the grand final with wins over St George and Wandoan-Taroom respectively.

The Gladiators thrashed the Saints 68-4, with most of the damage done in a sublime first half.

The Wallumbilla-Surat Red Bulls under-16s will join their reserve grade side next Sunday after beating Cities.