Police said Sewell was arrested on Tuesday, and is being questioned over an incident at a First Nations camp in Melbourne on Sunday.
Camp Sovereignty organisers said about 40 men dressed in black, some armed with flagpoles and sticks, violently attacked protesters after an anti-immigration rally.
Camp founder Robbie Thorpe told the ABC that four people were injured, including two who spent the night in hospital.
Police said they are also aware of an incident at a press conference with Victorian Premier Jacinta Allan on Tuesday, which two people interrupted.
They were heard shouting insults and accusations at Allan, whose security staff blocked the men.
Allan said she “never expected to come face-to-face with a neo-Nazi … at a press conference”.
Also on Tuesday, Sewell appeared in court charged with offences related to intimidating a police officer and breaching an intervention order in 2024.
MP Josh Burns said neo-Nazis had been on the steps of parliament.
“Seeing neo-Nazis cowardly walk around our streets in masks and in uniform, in black-cladded clothes, cowardly hiding their own identities, is completely unacceptable,” Burns said.
“Having neo-Nazis turn up to the press conference of the Premier and elected officials is completely unacceptable.”
Australian deportation rules mean Sewell could be sent back to NZ after facing criminal charges.
- RNZ