The 18-year-old was last seen at about 11pm on May 31, 2019, when he was ejected from the Cheeky Monkeys bar in town.
Police were alerted six days later, when he failed to return to his hostel and could not be found or contacted.
A large-scale search was launched, with only a hat believed to belong to Theo located, and investigators have since been trying to track down the person who last spoke with the teen.
At an inquest into his disappearance on Thursday night, counsel assisting the coroner James Herrington said a team had finally done that.
Fellow traveller Loic Spiess gave evidence via audio visual link that he and Theo had been communicating via WhatsApp - an encrypted messaging app - in the days before he vanished in 2019.
Theo's last message to Loic was sent at 12.56am on June 1, just six minutes before his phone activity mysteriously ceased.
Mr Herrington said it was the last message, that investigators are aware of, ever sent by Theo.
Loic's reply later that day was sent but not delivered.
"We've heard our last witness in the inquest ... there's some poignancy about that because it may also have been Theo's last contact," counsel assisting the coroner Kirsten Edwards said after Loic's evidence.
The inquest will not resume sitting until June 7, when counsel assisting the coroner will make their final submissions.
It will also hear submissions from a lawyer representing the Hayez family, and a lawyer for NSW Police.
State Coroner Teresa O'Sullivan will then hand down her findings on October 21.
Over 13 days of hearings between November and February, the inquest has heard evidence that places the teen on a steep headland below the town's famous lighthouse around midnight the day he disappeared.
Theo was tracked - using data gleaned from Google location services and his mobile phone connections - to a local sporting field after being ejected from the bar.
He then charted a route through the Arakwal National Park to the beach below the headland, before the data shows his phone climbing the steep ascent towards the lighthouse, before it stops transmitting.
Police have theorised Theo clambered up the beachside cliffs, dropped his phone, then fell and was swept out to sea, something his family says goes against the teen's sensible, risk-averse nature.
His friends similarly told the inquest Theo was kind and responsible, and didn't take drugs or drink excessively.
Other theories include that he was disorientated due to intoxication, tracking towards the famous Cape Byron Lighthouse, or trying to find a beach party in Cosy's Corner with an unidentified person.
Police this week announced a $500,000 reward for anyone with significant information about Theo's case.
