Wild footage has captured a brawl between Sharks and Roosters fans during Cronulla's 20-10 sudden-death win on Saturday night at PointsBet Stadium.
The fight, involving up to ten people, was sparked near the Channel Nine tent during play and has since circulated widely online.
Stewards and police were forced to intervene as fans in opposing jerseys traded blows.
Police eventually got control of the ugly situation as a Roosters supporter was helped up off the ground.
Police have made no arrests at this stage, but are looking into the dramatic incident.
'An investigation is underway after a brawl involving up to ten people broke out within the grounds of a football stadium in Cronulla last night,' a statement read.
Wild footage has captured an ugly brawl between Sharks and Roosters fans on Saturday night
Police have made no arrests at this stage, but are looking into the dramatic incident
'Police have commenced inquiries into the circumstances surrounding the incident.
'As inquiries continue, anyone with mobile phone footage or information is urged to call Crime Stoppers on 1800 333 000.'
Often derided for their poor September record, Cronulla overcome an early deficit and then hung on late to claim victory in a Saturday night elimination-final bellringer.
In doing so the Sharks ended the run of a red-hot Roosters side, who had come into the finals threatening to go deep as one of the form teams of the competition.
Cronulla will now travel to face the loser of Canberra and Brisbane next weekend off the back of eight wins in nine games, for a do-or-die semi-final.
The victory marked just Cronulla's second finals win in their past 10 outings, a run that stretches all the way back to 2018.
But after finishing the season in fifth there were few signs of finals jitters from Fitzgibbon's men on Saturday night, in a match that lived up to the billing.
'Everyone's story or journey has some highlight moments, but it has some pain in it. We've experienced a fair bit of pain in the past few years,' Fitzgibbon said.
Cronulla overcome an early deficit and then hung on late to claim victory in a Saturday night elimination-final bellringer
'I think we're different. I don't think anyone has noticed, but we'll shut our mouths and get ready for next week. It's not over yet.
'If we were ever going to get flustered then we would have tonight. We were nowhere for a long period there.
'Nothing went our way, no field position, no good ball. But we just didn't flinch. And they didn't either.'