Ms Martin’s son Leo Larsen, 13, and daughter Mia, 4, both died.
Their one-year-old son Ace is fighting for life in hospital.
Police say the family was travelling from Pimpama to Pratten, near Warrick, when the crash occurred on a relatively straight section of road in a 100km/h zone.
“The vehicle appears to have veered to the left and struck a three-strand wire road barrier before travelling across the road onto the opposite side and collided again with the barrier there,” Inspector Doug McDonald said.
“The vehicle has subsequently rolled down a stone embankment and came to rest on its roof in a semi-submerged position in the Wyaralong Dam.”
The car was removed from the dam on Sunday night and will now undergo forensic testing and a mechanical inspection.
Police say investigations into the cause of the crash are still at an early stage.
“Alcohol doesn’t appear on initial investigations to be a factor but we keep an open mind in relation to all the fatal five, and in particular, people travelling long distances,” Inspector McDonald said.
“Fatigue is a real big killer on our roads, and that’s certainly part of our investigation for this matter.”
Detectives are appealing for anyone who was travelling along Beaudesert-Boonah Road on Sunday about 2pm to come forward.
Ms Martin and Mr Wadely have undergone surgery for leg and pelvic injuries — they are otherwise in good health, but coming to terms with the loss of two beloved children.
Ace is still in a critical condition.
Pratten local Diane Lawlor told 9News she was “devastated”.
“I’m really— it’s a bit hard to say.”
She said the small community will be doing what they can to help the family.
“Certainly we’ll be handing round the hat to help this family and do what we can.”
Leo’s high school has offered support to his friends and classmates.