Both teams entered Sunday's contest unbeaten but reigning premiers Brisbane made it three straight wins to start their 2022 campaign as Hagiga Mosby's try with four minutes remaining proved the difference.
The 21-year-old crossed after St George had levelled the scores with 10 minutes remaining through a deft reverse pass from Quincy Dodd which allowed Elsie Albert to score her maiden NRLW try from close range.
The Papua New Guinean was impressive through the middle all afternoon as the Dragons dominated the second half only to be undone by Brisbane's lethal left edge.
With a 12-6 lead at the main break through tries from Tallisha Harden and Tarryn Aiken, the Broncos were pinned inside their own half to start the second as the Dragons' middle forwards made inroads.
And when Page McGregor found Shaylee Bent with a cut-out pass on their 30m line, the second rower showed a clean pair of heels to fend off and streak away to level the scores at 12-12.
Brisbane fired back shortly after with their first real opportunity of the second stanza through their right edge, going through the hands as Shenae Ciesiolka found Emily Bass who dotted down in the corner.
Albert's try levelled the scores at 18-18 following Rachael Pearson's conversion but Mosby was too strong in the corner late as Brisbane held on.
Bent and Emma Tonegato were both strong with their carries for St George Illawarra as the pair both finished with more than 150 run metres for the game.
For the Broncos, reigning NRLW Dally M player of the year Ali Brigginshaw combined well with halves partner Aiken to capitalise on the width their forwards provided.
But it was Amber Hall (19 runs, five tackle busts) and Millie Boyle who laid the platform for the Broncos' offence, finding numerous offloads to stretch their opponents and power Brisbane to their third win of the season.
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