A Sydney policeman who secretly filmed himself having sex with women has been found guilty of all charges.

Marc David Osborn, 42, was charged with three counts of filming a person engaged in a private act, for the purpose of obtaining sexual gratification.

The senior constable from Gladesville police previously told Downing Centre Local Court he showed the footage to his younger colleagues to prove he was "one of the boys".

Magistrate Janet Wahlquist says the videos were extremely explicit and were made to prove Osborn was a "competent and virile sexual performer".

The court was told the women were not aware they were being filmed and trusted the officer.