A spectator screening of the Matildas' clash against Sweden is under threat after more than 50 flares were set off at a Melbourne fan site.
Thousands of fans flocked to Federation Square on Wednesday evening to cheer on the women in green and gold in their history-making Women's World Cup semi-final against England.
However, the event was dampened by police having to contend with unruly young men in the crowd.
Officers were constantly trying to ensure fans' safety, but spectators breached temporary fences multiple times.
More than 50 flares were set off, with some thrown into the crowd.
A Victoria Police spokeswoman said she wasn't aware of anyone being injured.
"Overall, it was a challenging night for our members out at Federation Square, where we saw a large, rowdy group of mainly male youths, part of the active supporter groups, discharge and throw a significant number of flares," she said.
Police doled out fines of more than $960 to four males, aged between 16 and 23, for possessing flares, and cautioned another 16-year-old boy for throwing a flare.
Detectives are investigating the use of flares at Federation Square and have secured CCTV footage.
Police were otherwise happy with spectator crowds at bars and pubs around Melbourne and at AAMI Park.
"There were no incidents of note at AAMI Park, other than a penalty notice issued to a male who was in possession of a flare," the spokeswoman said.
Fans were expected to gather at Federation Square again on Saturday to watch the Matildas take on Sweden in a bid to secure third place in the World Cup.
However, Melbourne Lord Mayor Sally Capp suggested the event might not go ahead because of the behaviour at Wednesday night's screening.
"(It's) really disappointing," Ms Capp told Melbourne radio station 3AW.
"So if we could find those idiots and make sure they don't attend, then Fed Square will be going off again, but unfortunately, sometimes the few ruin it for the rest of us.
"We're looking at all of the logistics at the moment - the police are really having a good think and will give some advice, but there'll be lots of live sites for people to enjoy the football this weekend to watch those brilliant Matildas."
In NSW, fans endured long delays getting home from Sydney's Olympic Park on Wednesday night after a suspected act of vandalism threw the rail network into disarray.
The incident at Ashfield affected trains from about 10pm, about the time the semi-final, which England won 3-1, wrapped up in front of a crowd of more than 75,000.
Two men aged 47 and 33 were assisting NSW Police with their inquiries after officers were told cables in a railway signal box were cut and equipment was damaged.
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