02/09/2016 6:07 PM AEST | Updated 02/09/2016 7:52 PM AEST

Mystery Deepens: Tromp Daughter Found In Ute

'She did not know her name'.

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Riana Tromp was reportedly found in the back of a ute.

The mystery surrounding the whereabouts of Victorian man Mark Tromp has deepened, after the 51-year-old's adult daughter was reportedly found "distressed and disoriented" in the back of a ute in Goulburn, NSW.

Mark and his wife Jacoba "Coby" Tromp, were last seen together on Tuesday in the NSW Central Tablelands, when they became separated from their three adult children.

On Thursday Jacoba Tromp was found by a passerby and taken to a hospital in Yass, near Canberra.

In yet another strange twist, reports have emerged one of the couple's adult children, Riana Tromp, was found in the back of a utility truck in Goulburn on Friday.

After filling up with fuel at a Caltex Service Station in Goulburn on Tuesday and heading to Canberra for a doctor's appointment Goulburn resident Keith Whittaker received a shock as something knocked the back of his seat.

"I felt a kick to the back of my seat. I turned around and saw two legs stretched across the back between my seat and the floor. She was lying on the floor," Whittaker told The Goulburn Post.

"She did not know her name and had no idea where she was."

Mark and Jacoba Tromp.

Mark and Jacoba Tromp fled their home in Silvan, in Melbourne's outer east, on Monday with their three adult children.

Four of the family members were all found in separate locations, shell-shocked, while Victorian Police are currently searching for Mark.

On Thursday Mitchell Tromp, 25, told media the trip wasn't dissimilar to road trip in a scary movie.

Mitchell said he went on the trip to convince his parents to come home. The children were told not to bring their phones, but Mitchell brought his and eventually threw it out the window at his parents request, he said.

The 25-year-old told media his parents paranoia grew on the trip to a point where each adult child left.

"It slowly got worse as the days went by," Mitchell said.

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Mitchell and Ella Tromp.

"They were just fearing for their lives, and then [we] decided to flee."

The Tromp family have a berry farm and earth moving company in Victoria's Silvan, in the Yarra Valley.

Tromp's disappearance is not believed to be sinister. It was believed he was last seen following a couple in the Victorian town of Wangaratta in the family Peugeot.

Police have confirmed Riana has since undergone a mental health assessment and is in the care of mental health services in Goulburn with her mother who was found wandering and appeared agitated on Thursday.