Lisa Hayez said it would be completely out of character for him to have gone by himself for a late-night, cold swim in the ocean at Byron Bay or along a very dark bushy track to the beach.

"There is no way he was there by himself with no lights in the middle of bushland," she testified in the NSW Coroners Court on Tuesday.

Ms Hayez, who recommended he visit Byron Bay after she enjoyed its "special kind of vibe" and natural beauty, saw him off in Brisbane when he got a bus to the coastal tourist spot in late May 2019.

The 18-year-old backpacker disappeared after he was ejected from the Cheeky Monkey's bar at about 11pm on May 30.

After midnight, Theo used his phone to swap messages with his stepsister Emma, watch clips from a French quiz show, and joke with a friend on Facebook about an upcoming U2 concert.

But just after 1am, his phone stopped receiving data.

Police believe he clambered up the beachside cliffs, dropped his phone, then fell and was swept out to sea.

But his family, who have travelled to Australia for the two-week inquest, don't accept their cautious boy would do something so dangerous in the dark.

They say the only way he would climb the cliffs is if someone was with him, reassuring him it was safe.

Ms Hayez, who's 18 months older than her cousin, said they had been very close and in 2018 attended the same final-year class in Belgium.

She arrived in Australia as a traveller in September 2018 and caught up with her cousin when he arrived in November.

The good, organised student was planning to return home in June, to study for an exam to be admitted to university for an engineering degree.

The cousins lived together in Melbourne and regularly kept in contact when they later went on their separate travels.

They last spent time together on May 25 in Brisbane before he left for Byron Bay the next morning.

While he had really enjoyed his travels, he had been excited about returning home in June to start a new chapter in his life.

Theo was very good at navigating with Google Maps and had a strong belief he would never become an excessive or binge drinker.

After he disappeared, she and others walked from the Cheeky Monkey along the route, outlined from Google Maps data, said to have been taken by Theo.

They went along the dark track at night but even with others and torches "it was scary" and "no way" would her cousin had gone through the dense bushland alone.

The inquest continues.

Australian Associated Press