About 1.30am, emergency services were called to King Street, near Eliza Street, Newtown, following reports of a fight between two groups of men.
On arrival, officers found two men believed to be aged 46, one suffering head and facial injuries, and the other had been knocked unconscious.
They were treated at the scene before being taken to Royal Prince Alfred Hospital for further treatment.
Police believe three men fled the scene prior to the arrival of police.
Anyone with information is asked to contact Newtown Police Station on (02) 9550 8199 or Crime Stoppers on 1800 333 000.
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