Police were called to the fire at the Fenwicks Road property at about 12.20pm on December 20, where they found the bodies of the pair.
Olsen is in custody and is set to appear in court today after an initial appearance on Saturday.
With the investigation still ongoing, police have released an image of a white Holden Barina wagon, with a solar panel on the roof.
Anybody who may have seen the car from 10am to 2pm between Biggenden and Bam Bam Springs on December 20 is urged to come forward.
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A white Barina was found burnt out alongside a caravan and trailer following a police search previously.
Officers allege it was set alight the same day as the Mooney and Kirra died in the fire.
Anybody with information is urged to contact Crime Stoppers on 1800 333 000.