The former senior constable, 31, has pleaded not guilty to the murders of TV presenter Jesse Baird and flight attendant Luke Davies, saying both men were shot by his police pistol in self-defence.
An old friend of the officer who travelled with Lamarre-Condon when he first tried to dispose of the bodies began giving evidence to the 15-member NSW Supreme Court jury on Tuesday.
The woman, known by pseudonym Elizabeth Kennedy, met the accused as a teenager.
She told the jury that Lamarre-Condon was obsessive before he met Mr Baird.
"He would send me photos of celebrities and make out he was friends with them when he was actually just stalking them," Ms Kennedy told the court.
But it was once Lamarre-Condon began casually dating Mr Baird the officer's jealousy became evident, Ms Kennedy said.
"He was always worried about (Mr Baird) seeing other people ... he would go through his phone," she said.
"(Lamarre-Condon) called me from Monaco ... he said how annoyed he was Jesse couldn't see his (Instagram) stories."
Ms Kennedy is expected to say Lamarre-Condon drove her 200km south to Cella Farm on February 21, 2024, two days after Mr Baird and Mr Davies were killed.
The Crown alleges the senior constable left Ms Kennedy at the farm gate and then attempted to sink the bodies of Mr Baird and Mr Davies in a dam.
Ms Kennedy was left behind despite allegedly buying him the angle grinder he used to cut the farm gate's padlock at Bunnings Goulburn earlier in the afternoon, the jury has heard.
Lamarre-Condon unsuccessfully tried to sink the bodies using gym weights and quick-set concrete and later dumped the bodies outside a Greek monastery 30km away.
Before the bulk of her evidence, Ms Kennedy confirmed she gave an induced statement to police, protecting her testimony from being used against her in court.
"Her evidence ... may tend to prove she was an accessory after the fact to (the alleged murders)," Justice Natalie Adams told the jury.
Ms Kennedy has not been charged with any offences relating to the deaths of Mr Baird and Mr Davies.
Earlier, the jury was shown CCTV from February 20, 2024, the day after Mr Baird and Mr Davies were killed.
In the footage, Lamarre-Condon can be seen driving back from the Sutherland Shire where he had dinner with a different friend, having allegedly confessed to the killings to him the previous afternoon.
Multiple cameras track Lamarre-Condon's hired van's movements to Club Cronulla, a place he and his friend stopped at earlier in the evening.
The friend can be seen getting out of the Tesla and inspecting a skip bin in the bowling club car park, in vision played to the jury on Monday.
The Crown alleges Lamarre-Condon told the friend he needed to find a skip bin and stopped at the club when they noticed one while driving past.
The friend, who is yet to give evidence, is the same man who was with the accused when he bought a surfboard bag at a sports store two days before allegedly murdering Mr Baird.
Later that night, a camera at the bowling club captures a van parking in the car park before a figure opens two skip bin lids and loads items into the bins.
Neither the figure nor the van can be identified in the footage.
A man resembling Lamarre-Condon can be seen walking back towards his Balmain home at 11.55pm.
Meanwhile, footage of a police officer firing a test shot into Jesse Baird's bedroom door was released to media on Tuesday.
Lamarre-Condon contests the Crown allegation that he killed Mr Davies - Mr Baird's partner - by shooting him through the door with his police pistol.
The trial continues.
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