The search for a missing Belgian backpacker has resumed in northern NSW, as investigators try to piece together the 18-year-old’s last-known movements in Byron Bay.

Theo Hayez’s distraught father Laurent made an emotional plea for help on Monday, 17 days after he was last seen leaving the holiday town’s Cheeky Monkey’s bar about 11pm on May 31.

Mr Hayez said he feared his son was in grave danger and begged for any information that could lead police to his whereabouts.

“Before I left Belgium I promised Theo’s little brother Lucas that I would bring his brother home. Please, help me keep my promise to him,” Mr Hayez said through tears.

Theo used encrypted service WhatsApp the night he disappeared and Mr Hayez asked for help obtaining access to the messages.

Theo Hayez, who was last seen in Byron Bay on June 1. Photo: AAP

WhatsApp confirmed it had provided all the information it has to police.

However the company is able to access and disclose only some data – which can include a user’s name, last seen date, IP address and basic information – but not the encrypted content.

Dozens of people, including State Emergency Service volunteers and public volunteers, on Tuesday returned to the area around Cape Byron Lighthouse where the last ‘ping’ (a network query to determine whether there is a connection) from Theo’s phone was recorded on June 1.

He had been preparing to return home after eight months travelling Australia.

Homicide detectives are helping the investigation along with Belgian authorities. Police say they hold hope Theo is still alive.

More than $65,000 has been raised via a GoFundMe page for the Hayez family, part of which was used to help Laurent Hayez fly to Australia.

Mr Hayez arrived in the country late last week and has vowed to stay until his son is found.