Ben Apikatoa had responded to a call for help from Cassie Sanders, who was allegedly assaulted by her former partner, Nathan Chatimba.
Police allege the two men - who were known to each other - were arguing in the front yard of the St Clair house when Chatimba attacked Apikotoa.
Chatimba was arrested about two hours later at a home some 2km away and was charged with murder and assault occasioning bodily harm.
The matter was briefly mentioned at Parramatta Bail Court yesterday where Chatimba, through his Legal Aid lawyer, did not apply for bail and it was formally refused.
He is due back at Penrith Local Court on October 12.