St George-Illawarra, formed by a controversial merger this year, trailed Cronulla 8-0 at the break, with the only try coming from David Peachey.
But Mundine, who could face Great Britain in the Tri-Series next month, cut loose by scoring three tries and helping to set up the other for Luke Patten.
The Dragons will meet the Melbourne Storm at Stadium Australia. The Victorian outpost club upset the odds to beat Parramatta 18-16 on Saturday, reaching the Grand Final in only their second season.
A penalty by Chris Ross with two minutes remaining yesterday took Hunslet through to Saturday's Northern Ford Premiership Grand Final against Dewsbury Rams at Headingley.
In a mistake-ridden, nervy game at the South Leeds Stadium the scores were level at 8-8 when the Widnes forward George Mann was penalised for a robust tackle on Lee St Hilaire 20 metres out and Ross kept his cool to land the vital goal.