Penalties will vary between states but for NSW the maximum fines for selling games with restricted classifications to underage customers are:
. | MA15+ | R18+ |
---|---|---|
sold by individual | $5.5k | $11k |
sold by corporation | $11k | $22k |
https://www6.austlii.edu.au/cgi-bin/viewdoc/au/legis/nsw/consol_act/cfacgea1995596/s30.html
https://www8.austlii.edu.au/cgi-bin/viewdoc/au/legis/nsw/consol_act/cpa1999278/s17.html
These seem steep enough to encourage compliance.
But G, PG, or M? The customers age is none of their business and I wouldn’t expect them to take an interest.
Edit: to put those penalties in perspective the sentencing for supplying alcohol to minors scales up a maximum of $11k and/or 12 months incarceration.
https://www5.austlii.edu.au/au/legis/nsw/consol_act/la2007107/s117.html
While for tobacco the maximum penalties are:
. | first offence | subsequent offences |
---|---|---|
sold by individual | $11k | $55k |
sold by corporation | $55k | $110k |
https://classic.austlii.edu.au/au/legis/nsw/consol_act/pha2008178/s22.html
So it looks like it is penalized significantly more lightly than alcohol or tobacco.