It was the last time the 76-year-old father-of-three from Sydney was seen alive.
He earlier had been cleared to travel by his GP despite concerns from his family that he was showing signs of early dementia.
A coroner's report released on Thursday concluded it is impossible to determine how or exactly when he died.
Mr De Ornelas may have wandered off a defined walking track and become disorientated in the thick forest, coroner Madeleine Wilson said.
Mr De Ornelas, who immigrated from Portugal in 1973, regularly travelled with his wife, enjoyed the outdoors and often walked 8km throughout the course of a day.
The couple arrived in Tasmania in February 2010 on a trip organised by their local Portuguese community.
On March 3 they travelled with a tour group to Geeveston, south of Hobart, to the Tahune Airwalk - a 600m elevated walkway surrounded by bush.
Mr De Ornelas sat down next to another tour group member after completing the airwalk before saying he was going to head back.
After leaving the airwalk, he was seen down a narrow gravel road walking with purpose around 2-2.30pm.
He was reported missing shortly after 2.30pm, sparking a multi-day search involving ground crews, helicopters, divers and dogs.
There was no evidence of foul play in his disappearance, with Ms Wilson finding he died between March 3 and 8.
"Regrettably, without a body or direct evidence of his death, it is not possible to determine the exact circumstances around his death," Ms Wilson said.
She was unable to say whether Mr De Ornelas suffered a medical episode and noted he likely died of hypothermia, exhaustion and dehydration.
It highlighted the importance of staying on defined pathways and tracks in areas surrounded by bushland, she added.
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