Forensic pathologists who conducted the autopsies in Brisbane ruled out foul play.
Noelene Bischoff and her daughter Yvana died last month while on holiday in Bali.
Scombroid food poisoning combined with lingering asthma were identified as the key causes of death by coronial authorities in their briefing with the Bischoff family on Tuesday.
"They're sort of saying it's just one in millions of chances that [this] could all come together for that to happen with two people at the same time," Brother Malcolm Bischoff told the ABC.
He stressed that the results are preliminary. The finalised autopsy results will be sent to the state's coroner.
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Ms Bischoff and Yvana were farewelled at a funeral in the Lockyer Valley on January 20.
