Madonna exposed a fan's breasts on stage in front of thousands of people at a gig in Brisbane, Australia.The 57-year-old star invited the attractive brunette onto the stage and proceeded to yank off her strapless bra, which left one of her boobs exposed to the audience.The singer, who was 20 minutes late for the gig, began the bizarre incident by complimenting the woman on her appearance, saying: ''She's the kind of girl that you just want to slap on the a** and pull....''Madonna then removed the woman's corset-style top, before saying: ''Oh s**t. Oh sorry, sexual harassment.''She added, while gesturing to her crotch: ''You can do the same to me if you like...''The incident is the latest in a series of bizarre moments that have blighted her Rebel Hearts tour.The previous night, for instance, Madonna turned up on stage hours behind schedule and told the waiting crowd: ''It's you people that get here early, that's the problem.''The incident prompted show promoters Live Nation to warn fans that Madonna's latest on-stage performance was not certain to start at the advertised time.In a statement posted on Facebook, Live Nation explained: ''Madonna's Rebel Heart performance will proceed this evening in Brisbane. Following a late start last night, fans should note, the timing for tonight's performance may also be delayed.''The erratic behaviour has coincided with Madonna tussling with her ex-husband Guy Ritchie for custody of their 15-year-old son Rocco, who left her tour in December to be with his father.