Zulfikar Mohamad Shariff's detention under the Internal Security Act this month is his latest brush with the authorities and the law.
The controversial figure has agitated on Muslim issues in the past, and sought to sow discord.
He first made headlines in 2002 as head of fringe group Fateha. After Singapore announced the arrest of 13 members of the Jemaah Islamiah regional terror network early that year, he said the Government had prompted their intentions by "aligning itself so closely to the United States and Israel".
He sparked a confrontation that many Muslim Singaporeans were critical of, when he urged some parents to insist that their daughters attend school in tudung, or headscarves, contrary to the policy of having a common uniform.
Zulfikar hosted politicians from Malaysia's Parti Islam SeMalaysia and went to Malaysia to speak on the issue and draw international attention to it. His approach drew criticism from Muslim leaders.
After police began looking into possible criminal defamation of then Senior Minister Lee Kuan Yew and Minister-in-charge of Muslim Affairs Yaacob Ibrahim in Zulfikar's online postings, he left for Australia with his family.
He became a research fellow with Monash University, and led an organisation called the Association For Democracy In Singapore that purported to push for free speech.
It counted opposition figures such as J. B. Jeyaretnam, Singapore Democratic Party chief Chee Soon Juan and former Workers' Party election candidate Tang Liang Hong among its advisers.
In 2013, Zulfikar, who had enrolled in a PhD programme in international relations at La Trobe University, set up the Al-Makhazin website and linked Facebook groups to champion what he saw as issues affecting Muslims.
As it turned out, the site was a front to spread his radical agenda: to replace Singapore's democracy with an Islamic state, even as he remained in Australia, the Ministry of Home Affairs said yesterday.
When an Islamic religious teacher launched the controversial Wear White campaign in 2014 against homosexuality and the annual Pink Dot event by the lesbian, gay, bisexual and transgender community, Zulfikar was among the loudest supporters of Wear White.
After the Islamic State in Iraq and Syria (ISIS) proclaimed its caliphate that year, Zulfikar openly displayed support for the group and its military advances on Facebook, and radicalised at least two others.
He also uploaded a photograph of himself with five of his six children in front of a black flag associated with ISIS, mimicking a pose adopted by the group's fighters.
And he joined hardline group Hizbut Tahrir, which seeks to unify all Muslim countries into a caliphate.
He also kept company with radical preachers, including Australia's Musa Cerantonio, who has inspired foreigners to fight in Syria, and British-born Anjem Choudary, who faces trial in Britain over terrorism charges - while reportedly getting grants and benefits from the Australian government.