Raymond John Brookes mailed the four-page document to Sydney's largest mosque in Lakemba and independent Western Australia senator Fatima Payman in January.
Titled, "V for Vendetta," the often rambling letter called for revenge against Muslims, Aboriginals and Torres Strait Islanders, Labor, the Greens and "socialist" journalists at the ABC, SBS and the Guardian.
The Belmore man called for anyone who protested during Invasion Day rallies to be "beaten to death" in the street.
"You must do your bit for Australia," the 70-year-old wrote.
He sent two letters to Lakemba Mosque, one addressed to the Mullah and the other to the Australian Muslim Advocacy Network.
Lebanese Muslim Association secretary Gamel Kheir said the manifesto had created a sense of vulnerability and insecurity for members of the Muslim community in going anywhere in public.
"When places of worship of any kind, or other places like community halls, when they're targeted, it does strike fear," he told AAP.
He said Brookes had a lot to answer for, wanting maximum impact by targeting Sydney's largest, oldest mosque.
"To write a whole manifesto shows just the level of depravity and hatred that someone has for a community collectively," he said.
Brookes has pleaded guilty to three counts of sending a document threatening to kill.
He has yet to enter a plea to a fourth charge relating to a separate letter sent to NSW Labor MP Jason Clare in late 2023.
That document celebrated the "Muslim Brotherhood in Palestine" and called for hatred against the Jewish community, court documents seen by AAP allege.
Brookes appeared by video link wearing a prison-issued green top from Silverwater prison during a brief hearing at Bankstown Local Court on Tuesday.
Magistrate George Breton postponed the 70-year-old's sentence hearing to April 14 to get clarity on the charges, particularly the contents of the letter sent to Mr Clare.
The manifesto referred to Senator Lidia Thorpe, two nurses who allegedly threatened to kill Israeli patients, pro-Palestinian protests held on the anniversary of the October 7 attacks by Hamas, and the mass shooting at a Hanukkah celebration at Bondi Beach.
Brookes called for a total ban on protests, mass deportations and a ban on immigration.
The manifesto ended with a dive into numerology and how Australia Day in 2026 showed "rebirth," "regeneration" and "new beginnings".
Brookes' fingerprints were found on the letters he sent out, court documents say.
Police raided his Belmore home two days before Australia Day.
He was arrested, charged and placed into custody before bail was refused.
22°C