A suspended police officer has been convicted and must be of good behaviour for 18 months after he was found guilty of grabbing his colleague's breast following a Christmas party in Sydney's CBD.

Ronald John Tarlington, 50, who has been a police officer for more than two decades, attended a party at The Rocks on December 5, 2018, then went to another pub afterwards with several colleagues.

About 10pm, a female colleague was sitting on Tarlington's lap when he moved his right hand into her unbuttoned jacket and grabbed her breast, causing her to stand up and walk away. The interaction was captured on CCTV.

Last year, magistrate Paul Mulroney found Tarlington guilty of sexually touching another person without consent. He found the officer touched the woman "squarely on the breast area" and intentionally squeezed her nipple for a sexual purpose.