A reward of $500,000 is being offered for information that helps NSW Police solve the mystery of what happened to Belgian backpacker Theo Hayez.
It comes as an inquest is told of a "potential breakthrough" regarding his phone activity around the time of his disappearance on May 31, 2019.
The 18-year-old was last seen leaving the Cheeky Monkeys bar in Byron Bay.
Mobile phone data shows he searched for directions to his hostel but walked in the opposite direction, towards the Cape Byron lighthouse.
He has not been seen again.
Acting Commander Tweed/Byron Police District Brendon Cullen says police have exhausted most avenues of their investigation.
"This reward is being offered because there is a gap in what we know," he said.
"There is a gap between what happened between the nightclub and Tallows Beach.
"We want to know if anyone has any information that can fill that gap. We want to know what that information is."
Speaking from Belgium, Theo's father, Laurent Hayez, said "while we are now coming up to three years since Theo's disappearance, we believe this reward may be one of our last opportunities to find out what happened to Theo".
"A key question we are seeking to address is whether he was with someone on the night of May 31, 2019, which we strongly believe to be the case."
Theo's mother, Vinciane Delforge, stressed how "any detail, big or small, could help transform the investigation and provide our family ... with desperately needed answers".
"Not having answers means our family is not able to grieve and heal as we don't have a conclusion."
Police believe Theo may have fallen into the ocean and drowned while trying to climb a rocky cliff.
But a coronial inquest has heard 10 days' worth of testimony but uncovered no conclusive evidence.
Detective Acting Superintendent Cullen said he hoped the reward would prompt people to remember Theo's disappearance and provide information police might not yet know.
"This reward could change someone's life significantly if they have information that is useful to us; that's why it has been offered by the government.
"So if someone has that information, there is certainly incentive for them to come forward."
Inquest resumes
Today's offer of a reward for information came as the inquest resumed at the Byron Bay Courthouse.
Counsel assisting the coroner Kirsten Edwards told the court there could be some significant evidence to come this week.
"But we know that Theo was engaged in some WhatsApp activity ... just before that critical five-hour period when his phone stopped communicating with the tower.
"It was an area of intense focus by the police and by the family very early on in the investigation.
"A press conference was held, Home Affairs became involved, but nothing was able to be determined about that activity.
"We've found somebody who was communicating with Theo on May 31 [2019]; we're trying to ascertain whether that communication was close to that period just before 1am."
Ms Edwards said the court would also hear from Aruna Seneviratne, a professor of telecommunications at the University of New South Wales.
"It may be that we have been able to further refine where Theo's phone may have been located at around 1am or before that period of five hours where the phone stops communicating with the towers before coming back just after 6.15am, albeit very briefly."
Coroner Teresa O'Sullivan is expected to hand down her findings later this year.
