A teen gunman who shot and killed a police employee on Friday did so after he heard a lecture by the extremist group Hizb ut-Tahrir, Seven News has reported.

However, the group has downplayed the claim as "speculation".

Seven News said Farhad Khalil Mohammad Jabar was at a lecture given by Hizb ut-Tahrir at Parramatta Mosque on Friday, before he donned a black robe and killed finance worker Curtis Cheng outside NSW Police headquarters in Parramatta.

The 15-year-old then paced up and down the footpath and fired at armed officers before he was shot dead.

Hizb ut-Tahrir hit the headlines last year after footage surfaced of a 6-year-old boy at one of the group's youth meetings in Lakemba advocating for a caliphate and declaring "you're never too young to be a Soldier for Khalifa".

The footage, which was branded "sickening" by Channel Seven, spurred an investigation by police.

When Seven News reporter Brian Seymour knocked on the door of senior Australian Hizb ut-Tahrir figure Ismail Al-Wahwah, vision caught on a body camera showed Mr Al-Wahwah pointing his finger at Mr Seymour, repeatedly telling him to "get lost" and calling him a liar.

"The people pay to you to tell Australia, say the truth not to fabricate lies," Mr Al-Wahwah said.

Seven News were unable to confirm if Farhad was a member of a Hizb ut-Tahrir youth group or if he had just been there that day to hear the lecture, they said.

The gunman has been linked to an Islamist group already under investigation. http://t.co/MitOO7vBxV #7News http://t.co/TYkMqF93HR— 7 News Sydney (@7NewsSydney) October 4, 2015

A spokesman for Hizb ut-Tahrir in Australia, Uthman Badar would not comment to Fairfax Media on the existence of any links between Farhad and his organisation.

"Any links are speculation and we could caution against making speculation," Mr Badar said.

Hizb ut-Tahrir rejects democracy, secularism and all Western models of state. On Sunday it held a rally in Lakemba protesting against military action by Russia and the US in Syria.