The senior constable has pleaded not guilty to the murders of Jesse Baird and Luke Davies, suggesting the two men died after two separate struggles in Mr Baird's house.
His barrister said Lamarre-Condon entered the Paddington house in February 2024 armed with his police pistol and heavily disguised and did not intend to kill his former partner.
In footage played on Thursday to the 15-member jury, Lamarre-Condon can be seen entering a sports store in Sydney's south with his friend Ethan Lynch.
The senior constable picked up a surfboard bag and held it up for Mr Lynch to assess.
In the video, which had no audio when played, Mr Lynch moved towards the bag as Lamarre-Condon held it against his friend.
Lamarre-Condon returned the bag to the rack, picked up a bigger bag and paid for it using the same amount of cash he withdrew at an ATM the day before.
In his opening address, prosecutor Brett Hatfield SC said Mr Lynch was expected to give evidence saying the accused asked him whether he would fit inside.
"I'll grab this one instead, it should be big enough," Lamarre-Condon allegedly said when he picked another bag.
Mr Hatfield alleged he bought the surfboard bag to dispose of Mr Baird's body when he killed him two days later
However, the prosecution alleges the senior constable was surprised by Mr Davies during the killings, forcing him to adapt during his efforts to conceal the deaths.
Lamarre-Condon was allegedly captured on CCTV buying a second surfboard bag from the same store the day the two men were killed.
Mr Lynch is also the person Lamarre-Condon allegedly confessed the killings to in the hours immediately afterwards.
After arriving in a car he hired in his mother's name the day before, Lamarre-Condon allegedly asked Mr Lynch to not judge him for killing the two men.
When his friend asked if he was joking, Lamarre-Condon allegedly responded: "I wish I was".
Mr Lynch asked how he did it and his friend answered: "With a gun, pew pew," while making a hand gesture of firing a gun, Mr Hatfield told the jury during his opening address.
Footage from the day before the men died showing the new couple at an inner Sydney pub prompted emotion from the two men's mothers seated in the public gallery.
The officer and Mr Baird dated during the summer of 2022-23, with the latter man viewing it more casually than his accused killer.
Many witnesses, including friends and colleagues of Mr Baird, have told the jury about conversations they had with Mr Baird about his fear of Lamarre-Condon's controlling behaviour.
But the trial has also heard from colleagues Lamarre-Condon interacted with in the months before he entered Mr Baird's townhouse armed and heavily disguised to allegedly murder the TV presenter.
The trial is expected to last several months, with more than 100 witnesses expected to be called.
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