A Sydney police officer has told a court he secretly filmed himself having sex with women to prove himself to younger colleagues.

Senior Constable Marc David Osborn is facing three charges of filming a person engaged in a private act for the purpose of obtaining sexual gratification.

The 42-year-old from Gladesville Police Station told Downing Centre Local Court today that he bought a clock with a hidden camera online in August last year, a week before the first offence.

He said he was working with officers in their 20s who would brag about their sexual relations.

Osborn said he was worried his colleagues did not believe him when he discussed his activities outside of work, so he played them the secretly filmed footage to show he was "one of the boys".

The court heard Osborn met the women through online dating sites.

He said the women were never aware they were being filmed and his younger colleagues would laugh when they saw the footage.

Crown prosecutor Matthew Karpin told the court the case was not about male psychology.

He said the women came to Osborn's house expecting privacy, but they were exploited.

The defence case is that Osborn's openness about what he was doing suggests he did not think he had anything to hide, and he was not filming the women for the purpose of sexual arousal.

The magistrate has reserved her decision until August 13.