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Peter Faithful has been missing from his Bishopdale home in Christchurch since Saturday morning.
Gloriavale Leavers' Support Trust spokeswoman Liz Gregory said in the past two years Faithful has spent time in mental health facilities, and then with family in Christchurch.
Faithful is 62 years old with grey hair and a short grey beard.
He was last seen wearing jeans, a blue shirt and black shoes.
"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.