December's great in the southern two-thirds of the country - where
Sydney and Melbourne are. It's not so great in the top third. It's in
the Tropics. Very hot and very humid, with loads of rain, maybe even a
cyclone. Afterall, it's their "Wet" season up there. Best time for
northern third is from late April to October. But you can luck out and
get some fine weather up north during the "Wet". You may not get any
rain at all, or if there is, it'll be heavy but brief, leaving you most
of the day to enjoy, although it'll still be quite humid. That's not to
say you won't get rain in Melbourne or Sydney during December either.
They can get a bit of odd, stormy summer weather during summer, like
yesterday - practically flooded out in Melbourne.

Be aware that during the "Wet", beaches near the shore in the northern
part (north of Tropic of Capricorn) are unsafe due to poisonous
jellyfish. Outer reef okay though. And beaches in Cairns are yukky,
mostly mudflats. Head 20km north and then all along there toward Port
Douglas, to get fantastic beaches. Some beaches have jellyfish nets, so
stick to those if you come in December and head north and want to enjoy
the beach.

Other tips: can't stress sunscreen enough. Our summer sun can be
lethal. Ozzie Osbourne described his 20 minute attempt to get a suntan
during a visit to Perth, as akin to being f****** nuked or napalmed.

As a rule, we don't tip, although I keep hearing that goes on, mostly
in the east. Grrrrr. I'd rather you didn't, as we don't exactly want
this encouraged. We have lots of BYO restaurants (not the posh ones),
so you can save a bit if you want to enjoy a good Aussie wine, or beer,
while eating out, by hitting the bottle-o (liquor store) first.
Restaurants will charge something for corkage.

Use Sydney's ferries as much as you can, it's actually a great way to
sightsee the harbour area which is so pretty. Lots to see and enjoy
there. Melbourne too - they have these great old-fashioned trams but
they're real, useful public transit, not tourist traps.
Have fun. Hope you have a great time.