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A two-year investigation for RNZ has found the Exclusive Brethren Church is a major and secret client of controversial private investigators Thompson and Clark.
A privacy law specialist believes the chuch's use of private detectives to spy on former members could potentially lay it open to charges of harassment.
Investigative reporter Nicky Hager says the discredited private investigation company has dossiers on at least 20 former members of the Exclusive Brethren, based on street surveillance, monitoring of social media, checks in public records and information supplied by church leaders.
Professor Nicole Moreham from Victoria University told reporter Ruth Hill it could also have a damaging psychological effect on those being watched.