A teenager has had his eye socket fractured after a brawl involving up to 50 people spilled into the streets outside a pub in Sydney's south yesterday.

An argument is believed to have broken out during the broadcast of the UFC fight between Connor McGregor and Nate Diaz at the Ritz Hotel in Hurstville, with the brawl spilling out onto the street.

Police said they were called to the scene just after 5pm and witnessed a fight involving between 40 and 50 people.

They found two 17-year-old boys with injuries, and one was taken to St George Hospital for treatment of a fractured eye socket.

One witness said he saw the teen knocked out cold by what he has called a “one-punch hitter”.

The other teenager is reported to have sustained bruising to his forehead.

Police said they also arrested a 15-year-old girl at the scene for using offensive language.

She was then taken to Hurstville Police Station, where she has since received a caution.

The brawl broke out on the street outside the Ritz Hotel in Hurstville. (9NEWS) (9NEWS)

Police are investigating the incident. (9NEWS)

No arrests have been made over the assaults but police are in the process of reviewing security footage and expect to charge others with offences such as assault and affray.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.