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.