The intrigue and mystery surrounding the Tromp family has deepened as it emerged that daughter Riana Tromp was found near "catatonic" in the back of a stranger's ute.

The 29-year-old was found in the ute in Goulburn on Tuesday - the same day her family's lives began to unravel with her parents, Mark and Jacoba Tromp, going missing from a family getaway because of their increasingly bizarre behaviour.

Riana Tromp was found in the back seat of Keith Whittaker's ute, while he was driving, after feeling a kick to the back of the drivers seat.

He pulled over and called police.

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"Until the police arrived, she mostly sat and stared straight ahead as if she was catatonic. They arrived about an hour later and took her back to Goulburn Police Station," Mr Whittaker told the Goulburn Post.

"She was a well-dressed young woman and she offered to give me $50 for my trouble, but I said no thanks. I was just glad to help her."

A police spokeswoman told news.com.au Ms Tromp was taken to Goulburn Hospital, where she is having mental health checks. Her mother, who was found agitated in Yass on Thursday, is also in the hospital undergoing similar checks.

Mark Tromp is still missing, but has reportedly been seen near Bega trying to hitchhike. Police on Friday after afternoon would only say they are following up every sighting
If accurate, it would be the first sighting of Mr Tromp, 51, since he is thought to have fled his car - with the key still in the ignition - after a bizarre incident in Wangaratta, northeastern Victoria, on Wednesday night.

He went missing, along with his wife, on Tuesday after the couple went on a trip to NSW with their three adult children.

The trip was supposed to be to get a break from "pressure" they had been under, son Mitch Tromp said.

"I've never seen anything like it. It's really hard to explain or put a word on it but they were just fearing for their lives and then they decided to flee," the 25-year-old told reporters.

"It was a build-up of different, normal, everyday events - just pressure - and it slowly got worse as the days went by."

However, a clearer picture is emerging about the chaos the family were in before they drove away from their Silvan home, east of Melbourne on Monday.

The Daily Telegraph is reporting the family may have been planning to flee Australia because they were so convinced their lives were in danger.

"In their delusional state, the entire family was planning on going overseas with ... but they decided against it saying 'passports can be tracked'," a police source told the Telegraph.

"They thought the safer option would be going into a place where technology cannot reach them."

Mitch Tromp said on Friday his father "thinks people are after him. He's not in a good state of mind".

He said the family were trying to support each other as best as they could - but none of them had experienced anything like it before.

They left their berry farm in Silvan, east of Melbourne, in their daughter Ella's car. They drove north to NSW, stopping at Bathurst and Jenolan Caves, without any real plan or direction.

Mitch Tromp said it was supposed to be a technology-free break. That accounted for why the family members, Mitch, his two sisters Ella, 22, and Riana, 29, and their parents left their mobile phones at home - and why Mitch threw his out the car window.

Ella and Mitchell Tromp address the media at Monbulk police station after the disappearance of their parents Jacoba and Mark Tromp in NSW. Photo / News.com.au
Ella and Mitchell Tromp address the media at Monbulk police station after the disappearance of their parents Jacoba and Mark Tromp in NSW. Photo / News.com.au

There were signs of the chaos surrounding the family when police got to their Silvan home. The doors weren't locked. Bank cards were still there and documents were strewn throughout the house.

One senior Victorian police officer, Sergeant Mark Knight, said it was the most "bizarre" case he'd seen in 30 years.

Mrs Tromp was in hospital in Yass for much of yesterday after she was seen walking around in a confused state. It's believed she was taken to a mental health facility last night.

She had been able to give police several addresses in the Wangaratta area in Victoria, where police believe her husband is.

He is thought to have been involved in a baffling incident in the town on Wednesday night.

A young couple in Wangaratta saw a grey Peugeot station wagon following them around the town about 10pm on Wednesday night, while they were in their car playing Pokemon Go.

"I could barely see his headlights because he was that close to my car," the young man told Fairfax.

The car then stopped near Merriwa Park, and a man got out of the drivers seat and ran away, disappearing into the night.

His identity hasn't been confirmed - but police were confident it was Mr Tromp.

Officers and the dog squad searched the park, but failed to find him. The man had been alone in the car, and fled in such a hurry he left the keys in the ignition of the car.

Police said on Friday morning the search of the properties connected to the family had found no sign of Mr Tromp, who they admit may not want to be found.

Mitchell Tromp said while on the trip pressures about life were getting worse and his parents decided to flee.

"It was a build-up of different, normal every day events, just pressure and it slowly got worse as the days went by," he told reporters.

"The best way to describe it is like a movie. I've never seen anyone act like this or conduct themselves in this way.

"I've never seen anything like it - it's really hard to explain or put a word on it but they were just fearing their lives and then they decided to flee."

Daughter Ella, 22, told reporters on Thursday in Melbourne the general stresses of life "just got a bit too much" for her parents.

Mitchell added that while his mother does stress a little bit when life gets tough, his parents' actions were out of character.

"I've never seen my parents or anyone at all go to this extreme," he said. "They were a bit paranoid."

On Tuesday, the day after the family began their journey, the three children decided they did not want to continue on as their parents became more and more mentally distressed.

"I've never seen anyone like this or anyone conduct themselves in this way. I just really want my dad to be found," Mitchell Tromp said.

"He's not dangerous, he's my mate, my father. I love him."

At a media conference on Thursday afternoon, Sergeant Mark Knight described the situation as a "very unusual case".

It was the most bizarre he'd seen in 30 years policing, he said.

WHAT WE KNOW

• Mark and Jacoba Tromp and their three adult children Mitchell, Ella and Riana leave their Silvan home on Monday, August 29

• The family left in Ella's grey Peugeot and headed to NSW

• On Tuesday morning, Mitchell leaves the group near Bathurst and makes his way to Sydney and then catches a train to Melbourne

• Ella, Riana and parents make it to Jenolan Caves, about 150km west of Sydney

• At the caves, on Tuesday, the daughters leave their parents and make their way to Goulburn

• Ella and Riana split up at a Goulburn service station because they want to get home different ways

• Riana found under mental stress on the side of a road near Goulburn and stopped by police in her attempts to go home.

• Riana taken to hospital, where she remains

• NSW Police informs Victoria Police about the family

• Ella acquires a car, drives home and arrives at the family's Silvan farm on Tuesday night while Victoria Police are there

• Mitchell arrives in Melbourne on Wednesday morning. At one point on the trip Mitchell admitted to having his mobile phone, but he voluntarily threw it out the window at Warburton.

• Ella's Peugeot found in Victoria, in Wangaratta, on Wednesday night

• A man, believed to be Mark, seen running from the car.

• On Thursday, Jacoba Tromp is taken to Yass hospital by a passer-by who sees her in the NSW town

• Extensive searches of Wangaratta fail to find Mark Tromp

WHAT WE DON'T KNOW

• Where Mark Tromp is

• How Mark and his wife became separated

• What the parents' movements were after daughters left them

• Why the family was "emotionally traumatised"

• Whether it was Mark seen running from Ella's car in Wangaratta

- with AAP