Richard Marshall’s Championship part-timers have been in indifferent form all season, but with eight regular starters on the sidelines they made a mockery of the form book with easily their best performance of the year.
Fax led 12-6 at the break and then withstood a London surge early in the second half, which saw the Broncos level at 12-12, before finishing the job with spectacular tries from Reece Chapman-Smith and Quentin Laulu-Togagae.
The home side took the lead in the 16th minute when Londoner James Woodburn-Hall kicked high for back rower Ed Barber, who out jumped full back Matthew Fleming to claim the ball before cantering the final 10 metres to score.
Centre Steve Tyrer added the conversion for a 6-0 lead and Fax continued to look dangerous in possession as the game entered the second quarter.
The Broncos, prompted by half back Morgan Smith, carried plenty of threat of their own in attack, with only a last-ditch ball steal from Tyrer keeping the home line intact.
The pressure finally told though, with prop Mark Ioane charging over with 25 minutes on the clock, Smith levelling the scores with the conversion.
Halifax weren’t done though and after Woodburn-Hall had raced 60 metres down the sideline they shifted the ball right, where Barber’s offload and slick hands from James Saltonstall and Ben White slipped Ben Kaye over the line.
Tyrer converted for a 12-6 interval lead that belied the home side’s recent troubles.
The Broncos were level eight minutes into the second half, with winger Jacob Ogden scoring in the corner after a sustained spell of pressure, Smith converting.
But when Fax got an attacking chance, they took it; White and Laulu-Togagae shipping the ball sharply to the right and Reece Chapman-Smith flying over by the flag.
Tyrer converted from touch for an 18-12 lead with 55 minutes on the clock and when James Saltonstall stormed up the middle with 16 minutes to go, Laulu-Togagae backed up on the inside to score the home side’s fourth try, Tyrer converting for a clear two-score lead.
The Broncos gave themselves a chance when Ogden went over in the corner, but Smith’s missed conversion left the gap at 24-16 and when Shaun Robinson pushed Ogden into touch as he dived for his hat trick, Fax were in the sixth round.
Halifax: Laulu-Togagae; Chapman-Smith, Tyrer, Saltonstall, Robinson; Woodburn-Hall, White; Kavanagh, Kaye, Fleming, Larroyer, Barber, Calcott. Subs: Kidd, Morris, Davies, Cooper
Broncos: Fleming; Ogden, Morgan, Kear, Helliwell; Smith, Meadows; Richards, Pelissier, Mason, Pitts, Gee, Lovell. Subs: Abdul, Cunningham, Battye, Ioane.
Referee: L. Moore