NSW police sergeant accused of dodging RBT

An off-duty police sergeant declined to be randomly breath tested and drove off after telling a junior colleague it would put him in an "awkward situation" if she blew over the limit, a Sydney court has heard.

Sarah Louise Johnston, 50, is on trial for using her rank and authority to avoid a breath test with the intent to pervert the course of justice.

She'd had at least one schooner of beer while out celebrating the new year with colleagues from North Sydney Police Station at two nearby pubs on January 8, 2016, the crown prosecutor said in her opening address at the Downing Centre District Court on Tuesday.

Johnston was driving home to the Central Coast when she was pulled over for a random breath test on the Pacific Highway at Crows Nest.

"You're not going to breath test me are you?" she allegedly asked then probationary constable Tugcan Sackesen.

"Yes sergeant I am," he replied.

She said, "No that would be a conflict of interest."

"Imagine if I blew over, which I won't."

The crown alleges Johnston told Mr Sackesen it would put him in an "awkward situation".

"It's a random breath test so you don't have to test everyone," she allegedly told him before driving off without being tested.

The crown said Johnston "was concerned about... the risk and possibility of failing the test, which is why she acted in the way she did".

Later that night Johnston told another sergeant about the incident, saying she'd "declined" an RBT and given Mr Sackesen "a lesson on RBT and in-the-job etiquette".

She also said she was "relieved I didn't have that last beer".

Johnston's defence barrister Raymond Hood said his client didn't intend to pervert the course of justice.

He said Johnston and Mr Sackesen only had small differences in their versions of the conversation, but one word could change a sentence's intended meaning.

He questioned the accuracy of the junior police officer's version of events.

"Did he get it right, did he get it wrong, has he supplemented the conversations from time to time?," Mr Hood said.

The trial continues before Judge Christopher Hoy.