Sosiua Tonga, 34, allegedly assaulted another 34-year-old man early on Sunday morning at an Eastern Creek house, resulting in a serious head injury
7News reports the injured man was taken to Westmead Hospital but died shortly after.
Tonga, who was arrested at the scene, was refused bail on murder charges.
On Monday, Tonga made no application for bail and it was formally refused during brief mention in Mount Druitt Local Court.
Tonga allegedly attacked the victim after he caught him at his girlfriend's home.
According to police, the two men did not know each other, but both knew the woman - named as Astrid Muller - in an apparent love triangle.
It's also believed Tonga is married, with a wife in Melbourne.