The 24-year-old Packer pled guilty to assault occasioning actual bodily harm and of failing to quit a licensed premises in a Sydney court on Monday and was given a custodial sentence.
Packer punched a man in the face multiple times and stomped on his head as he lay motionless on the ground.
The victim suffered two fractured bones to his face.
Packer's lawyers immediately filed an appeal and asked for bail, which was denied and the visibly surprised former New Zealand international was handcuffed and taken into custody, Australian Associated Press reported.
Magistrate Greg Grogan said the 24-year-old's behaviour was cowardly and deplorable, and sentenced him to two years' jail.
His appeal will be held on February 11.
Packer played for the New Zealand Warriors in 2013 and had joined the Newcastle Knights on a four-year deal, though the club had yet to register his contract.
The NRL has suffered numerous negative public relations incidents in recent years with players involved in allegations of assault and anti-social behaviour.
The league was also implicated in a wide-ranging investigation into drug use in Australian sport last year.
The NRL provisionally fined the Cronulla Sharks A$1 million ($898,200) last month and suspended coach Shane Flanagan for a year for exposing players to significant risk through their supplements programme, which was investigated by anti-doping authorities.