A leader of a hardline Islamist group which campaigns for sharia law says Muslims who leave the religion should be put to death.

Hizb ut-Tahrir spokesman Uthman Badar was frank when asked about the group's policy at a forum in Bankstown, in Sydney's south-west, on Saturday night.

'The ruling for apostates as such in Islam is clear, that apostates attract capital punishment and we don't shy away from that,' Badar said in the presence of children. An apostate is someone who decides to leave Islam.

His extraordinary admission was exclusively captured on camera by Daily Mail Australia and the matter has now been referred to the Australian Federal Police by Justice Minister Michael Keenan.

Hizb ut-Tahrir Australia removed references to that apostasy policy from its website as Alison Bevege, a freelance journalist, sued the group for making her to sit in a women's-only section at a separate talk in October 2014.

On Saturday night, Ms Bevege held up a printed copy of Hizb ut-Tahrir's draft constitution of the khilafah state published on the UK site, which was on the group's Australian website until 2015.

This outlines their vision for a global Islamic caliphate, which has Muslims and non-Muslims living under sharia law.

She asked about their policy of killing people born as Muslims who leave the faith.

Article 7c of the document said: 'Those who are guilty of apostasy (murtadd) from Islam are to be executed according to the rule of apostasy, provided they have by themselves renounced Islam.'

Badar initially responded by saying the policy wasn't on its website before explaining how the group's apostasy policy was compatible with Islam.

'The whole thing covers different aspects of Islamic sharia law,' he said.

'The role of apostasy in Islam is very clear. Again, this is one of the things the West doesn’t like and seeks to change the role of apostasy.'

A spokeswoman for Justice Minister Michael Keenan condemned language that incites or advocates violence.

'Language that incites or advocates violence is not freedom of speech,' the spokeswoman said.

'This matter has been referred to the AFP.'

Badar's remarks came after he delivered the keynote lecture for the forum, which was called 'Sharia and the modern age'.

He said Islam was incompatible with a secular separation of religion and state, democracy, individual rights and even the process of science, which he called 'scientism'.

He compared calls to fit Islam within a secular society to domesticating a wild animal, putting Hizb ut-Tahrir at odds with secular Muslims who reject sharia law.

'The West seeks to domesticate Islam, to control, to bring within, the way you domesticate animals,' he said.

Badar described calls to reform Islam from secular Muslims as 'pernicious', 'insidious' and 'dangerous' and called for radical change.

'Always when you hear these sorts of calls, alarm bells should ring,' he said.

'The Islam people are calling for fits very well within modernity. They’re giving in to the pressure to conform.'