Australian authorities offered an A$1 million reward on Saturday for information leading to the capture of a gunman on the run for 11 days after allegedly killing two police officers.

More than 450 police have been searching the mountains of Victoria state for 56-year-old Dezi Freeman, previously known as Desmond Filby, since the August 26 shootings on a rural property that also injured a third officer.

The reward is a record for the state, police said.

Freeman is alleged to have fired on a team of 10 police officers, including members of the sexual offences and child investigation team, when they arrived at his rural property to execute a search warrant.

Believed to have expert bushcraft skills and multiple powerful firearms, Freeman is described by local media as a "sovereign citizen" who regards the government as illegitimate.

"As part of the effort to locate Freeman and bring him into custody peacefully and safely, police are offering a reward of up to $1m for information on his whereabouts that leads to his arrest," Victoria police said in a statement. "This reward represents the largest ever offered in Victoria for an arrest."

Freeman, thought to be in remote high country, may be alone or have assistance, or could be "dead as a result of self-harm", police said.

The search area includes the town of Porepunkah, about 300 kilometres northeast of state capital Melbourne, where Freeman is alleged to have run into the bush in wintry weather.

Victorian Premier Jacinta Allan last month paid tribute to the two slain officers, Detective Leading Senior Constable Neal Thompson, 59, and Senior Constable Vadim De Waart, 35, amid what she said was the "huge operation" by authorities to catch Freeman. (Reuters)