Police believe Anthony Fane killed her on or around February 9.
The defence argues Anthony Fane was solely responsible for the Ormsby Lane shootings two days later.
In court yesterday, the jury heard about a phone call made from the Auckland South Corrections Facility, where third Fane brother Cody was an inmate, the day after the alleged murders of Lasslett and Littlewood.
The court was told it was made from Cody's cell.
Detective Daniel Elliott said the call was between the three Fane brothers: Cody, Samuel and Anthony. He played the recording - more than an hour long - to the jury and supplied a transcript he wrote.
In the call, Cody and Samuel discuss what happened on February 11 and 12, and the alleged motive for the killings. Then Anthony dials in.
Samuel tells Cody he's on a "grand tour" and had arrived in "H-town" (Hamilton) about 2am that day and he and Tarei had all their gear in their car.
The pair also discuss that their brother Anthony believed his partner and Lasslett were having an affair after he had been tracking her bank accounts and movements.
Samuel said: "... It is what it is ... the bro did it. There will be a time when it will all comes out. He [Anthony] was on a mission mate ... He couldn't handle it."
Anthony tells his brothers he "feels good" and plans to keep going "hard out".
"I'm proud of myself ... I'm on a little mission. No one is going to pick on me and I'm just going to carry on, " he says.
Samuel or Cody tells Anthony, "I'm proud of you too."
Anthony also tells his brothers: "Honestly it feels good ... I'm good bros, I am happy I didn't get anyone else involved brother. It came from me.
"It was easy bro. It made me f***** happy brother ... I never seen anyone so scared when I looked into his eyes ... I'm f***** happy I did it all."
After Anthony disconnects, Samuel tells Cody their brother had a big smile on his face once Anthony allegedly carried out the shootings "like he had won the Lotto, bro".
The brothers can be heard laughing and giggling throughout the call and several times say they love each other.
Detective Kristen Edginton, who conducted a scene examination at the Ormsby Lane address the day after the shootings, also gave evidence yesterday.
Edginton said he had done a cursory search near the entrance to the dwelling before entering to conduct a thorough search.
The jury earlier heard that a rifle was found hidden in a bush about 400 to 500 metres up Ormsby Lane from the property.
He confirmed a loaded Magnum handgun was also found upstairs on the mezzanine floor in Lasslett's dwelling and methamphetamine was found in the blood of both deceased.
Trace elements of a substance that appeared to be methamphetamine were found in bags and a P-pipe was located behind Lasslett's head, he said.
The trial continues on Monday.