is an exceedingly common Australian
acronym standing for Bring Your Own (as in, alcoholic beverages).
Many Australian restaurants or cafes without liquor licenses would advertise themselves as being BYO. This generally means that a bottle of wine can be enjoyed at a lower cost than for a licensed establishment, although many restaurants now charge a corkage fee of a couple of dollars per bottle.
Suburban get-togethers or barbeques might be termed "BYOG" by the (usually male) host. This means Bring Your Own Grog.