Sewell and three other men, Jake Crockett, Michael Saarinen and Nathan Bull, entered their guilty pleas in the County Court on Wednesday. Each said "I contest the facts" after entering the plea.
The incident occurred after an anti-immigration protest in Melbourne's city centre. The men, dressed in black, allegedly held down occupants of the Indigenous camp before kicking and punching them. Five people were injured.
Sewell's lawyer, Dermot Dann, KC, said witnesses would be called to give evidence in a pre-sentence hearing where contextual facts would be contested. "Antifa and protesters causing violence to cars – that's the context," Dann said.
Outside court, Sewell told reporters he did not dispute "spontaneous violence on the day" but argued it had causes. "What is in dispute is whether that spontaneous violence had causes and whether there was cause and effect of this," he said. Sewell said his group was provoked and suggested the average Australian would have responded the same way. "What must be contested is that this was not an attack on Aboriginal people," he said.
Bull has also pleaded guilty to a second charge of failing to provide his name or address to a police officer. Five other men have admitted to their roles in the attack and will face a pre-sentence hearing in November. Two others will contest their charges at trial.
Sewell and the three co-accused will return to court on November 9.