McLachlan is suing both media organisations and actress Christie Whelan Browne for defamation in the NSW Supreme Court over claims in 2018 reports of assault, indecent assault, harassment and that he exposed himself to female cast members.
The Gold Logie winner is also suing over claims about his conduct in workplaces beyond The Rocky Horror Show.
On Monday, McLachlan's barrister Kieran Smark SC described the trial as very significant for his client, saying it's a case about a "double- pronged attack made on him by two powerful media organisations".
In his opening address to the jury, Mr Smark said hundreds of thousands of people in Australia saw the strong attack on McLachlan via the ABC's 730 program and on the Sydney Morning Herald's front page, as well as online.
The barrister told the court that McLachlan, prior to playing The Rocky Horror Show's Frank N Furter in Brisbane, Perth, Adelaide and Melbourne in 2014, was a successful theatre and television actor.
He had prominent roles in soap operas Home and Away and Neighbours, before going on to perform in a stadium show of Grease in 2005, alongside Whelan Browne.
McLachlan later starred in the ABC television series The Doctor Blake Mysteries, which the court was told would also be important in the case.
Mr Smark tendered to the court three "staged" photos from the 2005 Grease production showing McLachlan in rockabilly costume and Whelan Browne in a cheerleader outfit. One the barrister said showed the pair "in the throes of passion" and another he claimed showed them "clearly simulating" sex.
The jury was also shown a short promotional video from the 2014 Rocky Horror Show production as well as excerpts from important "on stage" parts of the show, including the opening scene of Act Two referred to as the "Janet bed scene".
"It's a story about sex, comedically told," Mr Smark said.
The court was also told of photos from the production - which concerned cross-dressing and sexual themes - showing McLachlan in "female gear" like high-heeled shoes, fish net stockings, female underwear, a garter belt and a corset.
Whelan Browne, as shown in photos, performed at times in a white outfit, white bra, and white headband and at others in a "state of undress" in a red-and-black costume with a feather boa, the court was told.
"That's just how the show was," Mr Smark said, describing the production as "high energy" and its costumes as "skimpy" and "risque".
He said there would be evidence from McLachlan about how he got on with Whelan Browne and others in the cast of the musical, which he described as unfolding each night in the same fashion according to rehearsals and choreography.
Also on Monday, the court was shown a 12-minute "goof reel" made by the ABC production department before the 2014 season of The Doctor Blake Mysteries, which Mr Smark said included a carrot sticking out of McLachlan's trousers.
This, the barrister said, was "clearly intended" for comic effect, saying that McLachlan would give evidence about the "atmosphere on set" on the show.
He said he expected McLachlan to testify about the "camaraderie" on the show and how actors dealt with stresses was different to other workplaces.
McLachlan is expected to testify on Tuesday.
