The Fallout series has always had a hard time being reasonably rated in Australia. Previously in Fallout 3, the game had to remove references to morphine and other real-world drugs because the certification board took issue with their inclusion in a video game, forcing the studio to rebrand morphine as "Med-X". It was refused classification the first time it was submitted.
In any case, it looks like Fallout 4 won't be having such a hard time down-under as it's just been officially certified MA15+ by the Australian Certification Board.
According to the official page for the MA15+ rating, the game was rated as such due to its inclusion of "Violence" under Strong Impact. I guess people getting blown up by large explosive weapons would do that for the game.
The rating also notes "Themes, Language, Drug Use," and "Sex" under Moderate Impact.
The game will be out on November 10 for the PC, Xbox One and PS4.