links.mimuki.net

In short, in the context of sex:

M:

• Not recommended for under 15s, no legal restrictions
• Sexual activity may be "discreetly implied, if justified by context"

MA:

• Considered unsuitable for under 15s, with legal restrictions
  • I definitely bought MA video games as a teen without issue (though sometimes I did need my dad there)
• Sexual activity may be implied
• Sexual violence may be implied, if justified by context

R:

• Considered unsuitable for under 18s, with legal restrictions
• Sexual activity may be simulated (i.e. the actors are not actually having sex)
• Sexual violence may be implied

X:

• Exclusively used for consensual sexual material (but nonsexual, high impact films can be R or RC)
• Illegal to possess in some parts of the NT (maybe? I think that law automatically sunset a few years ago and wasn't replaced?)
• Illegal to sell, rent, or exhibit in all states (but not the territories)
  • Thanks to the constitution, an ACT/NT store can knowingly sell to someone in other states and that's OK
• Sexual activity may be depicted, unless...
  • depictions of violence, sexual violence, sexualized violence, coercion, or sexually assaultive language is present
  • that includes consensual depictions which "purposefully demean anyone involved in that activity for the enjoyment of viewers"
  • body piercing, "application of substances such as candle wax", "golden showers", bondage, spanking, or fisting are depicted
    • Why is this list so specific?
  • the actors are 18+ but look like minors, or are being depicted as minors

RC: (Refused Classification)

• Anything that fails the above restrictions, but also:
  • "activity accompanied by fetishes or practices which are offensive or abhorrent"
  • "incest fantasies or other fantasies which are offensive or abhorrent"
    • Why pretend-incest is singled out here but not in the exclusions of X ratings is beyond me.
• Broadcasting or selling this, or anything that would be RC if they tried to get it classified, is illegal (for the person selling/broadcasting)
  • I am fairly sure consuming or possessing it is not inherently illegal, unless you're in a juristiction where X-rated content is illegal to possess

See also:
https://www.legislation.gov.au/F2005L01284/latest/text