"He is not thought to have taken any food or belongings with him," Gregory said.
"He does not have a mobile phone and has no access to money. Faithful is mentally unwell and there are serious concerns for his welfare.
"Faithful suffers from selective mutism, and might not talk to strangers to request assistance."
Gregory said Faithful has an affinity for remote areas, and it's possible he may be in the wider forested regions of Christchurch, or he may have started walking or hitchhiking towards Greymouth.
He has been reported missing with the police.
If you know where Faithful is, contact the police on non-emergency number 105.