Christie Whelan Browne, one of McLachlan's accusers, has been named in the defamation suits, 9 News reports.
According to reports, McLachlan's statement of claim was lodged in the NSW Supreme Court and targets Browne.
The claim states Browne was 'herself a notoriously foul-mouthed person who publicly distributed offensive matter and had expressed interest in deviant sexual practices,' according to Seven News.
News Corp reports that McLachlan has engaged prominent barrister Stuart Littlemore QC to run the case.
A Fairfax Media and ABC joint investigation in early January reported several cast members of the 2014 run of The Rocky Horror Show accused the 52-year-old actor of sexual impropriety.
McLachlan has previously claimed the allegations were 'made up', and had vowed to fight the allegations.
Fairfax Media and the ABC on Thursday told AAP they wouldn't be commenting on the legal action.