The movie's stars - including Seann William Scott and Jason Biggs - joked around with fans and paparazzi during a photocall outside a cafe in Melbourne.
According to The Daily Telegraph, Scott, who plays Stifler in the comedy sequel, borrowed a camera from a nearby photographer and also laughed when a fan exposed his bare bum.
Biggs was spotted joking with on-screen dad Eugene Levy and replied, "It's good, thanks" when a fan asked him: "How is your d**k?"
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Other cast members, including Tara Reid, Mena Suvari and Chris Klein, also walked the red carpet. They then all sat through the first screening of the film, for which they received a standing ovation.
Shane Warne and ex-wife Simone Callahan both attended the premiere, their first joint public outing since he got together with Liz Hurley. Callahan walked the red carpet with boyfriend Toby Roberts.
Celebrity Apprentice Australia star Warwick Capper, actress Kimberley Davies and comedian Peter Helliar also appeared at the event.
Biggs was recently photographed taking a big bite out of a pie to promote the movie, which opens on April 5 in Australia.
American Reunion will debut on April 6 in the US and on May 2 in the UK.
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