Serious concerns are held for the whereabouts of a Belgian backpacker who has been missing for more than a week, after he failed to return to his hostel in Byron Bay.

Theo Hayez, 18, was reported missing on Thursday by staff at the Byron Bay WakeUp! Backpacker hostel, when they noticed he had not checked out of his room.

Theo Hayez and his cousin Lisa Hayez, in Sydney for New Year's Eve 2018.

Mr Hayez' cousin Lisa Hayez arrived in Byron Bay on Saturday morning and said her family were "really worried," waiting for "any news".

"His Mum texted me on Thursday morning telling me that she was really worried because he wasn't answering, and the hostel called the police on Thursday, as he was supposed to check out three days before that."