A former police officer accused of using his work gun to kill a Sydney couple was driven by a "jealous infatuation" with his ex, a court has heard.

Beaumont Lamarre-Condon, 31, has pleaded not guilty to two charges of domestic violence-related murder and one of aggravated break and enter.

Prosecutors allege Lamarre-Condon shot his ex-partner, 26-year-old TV presenter Jesse Baird, and Baird's new partner Luke Davies, a 29-year-old Qantas flight attendant, at a home in the Sydney suburb of Paddington in 2024.

The couple's bodies were found in a rural area 200km away, hidden in surfboard bags. The trial is expected to run for three months.

In his opening address on Monday, prosecutor Brett Hatfield SC described how Lamarre-Condon had developed an "obsessive and jealous infatuation" with Baird after a brief romantic relationship.

It manifested itself in a "range of compulsive behaviour", Hatfield said, which escalated when Baird ended all contact in November 2023.

The barrister said Lamarre-Condon used a friend's phone to message Baird and claim he had cancer, stalked his ex-lover, and tampered with his car in the weeks after their break-up.

On the day of the alleged murders, Lamarre-Condon had set his alarm for 04.30 local time, Hatfield claimed, and sent a text to a friend which read: "I'll try to call you tonight and update you... there's no sign I'm doing it".

Lamarre-Condon then switched to a burner phone, picked up a rental car and waited outside Baird's home for three hours dressed as an Uber driver, the prosecution alleged. He had previously bought a surfboard bag to dispose of Baird's body, Hatfield said.

After entering Baird's home with a key he had previously obtained, Lamarre-Condon shot Baird in the kitchen and then killed Davies in the bedroom as he was trying to call emergency services for help, the court was told.

Hatfield said that the accused had planned Baird's murder, and killed Davies so as to "eliminate him as a witness".

The aggravated break and enter charge that Lamarre-Condon is facing refers to an incident in August 2023 where it's alleged he broke into Baird's home and stole his phone and wallet.

Hatfield also told the court that Lamarre-Condon had earlier offered to plead guilty to a single charge of manslaughter relating to Baird's death but this was rejected.

Police were first alerted to the alleged murders after bloodied items belonging to Baird and Davies were found in a bin in Sydney's south.

Later that week, Lamarre-Condon handed himself in to a police station and was charged.

Eight days after the alleged killings took place, authorities found the couple's bodies.

Lamarre-Condon was sacked by New South Wales Police in the weeks after he was charged with the alleged murders.

Before the opening address on Monday, a 15-member jury was selected, with the judge, Justice Natalie Adams, explaining that three extra people were chosen - a jury is typically made up of 12 people - as a contingency for sickness and other unforeseen circumstances.