I'm English and have been living in Oz for three years now with my family. I am also a traditional Christmas fan so it took some adjustment for me to actually feel Christmassy!
Here in Australia, people love Christmas just as much as they do in the UK but it's far less comercialised. Yes there's more stuff in the shops but not nearly as much as in the UK.
It will be hot. Very hot. Since you're in a hotel, it's slightly different to being at someone's home. Most people spend the day outdoors, by the pool, in the garden or at the beach.
Food tends to be typically summer food combined with traditional English Christmas stuff. So many people have roast turkey and potatoes but it's also going to be offered with good shellfish, extra-nice salads and lovely fruit like strawberries, cherries and mangoes. People love pavlova at Christmas too.
I think that i would plan for using the pool a fair bit on the day OP...there is one isn;t there?
For gifts it's a bit trickier. Before we lived here, we used to come for Christmas with DH's family who live here and we'd do our Christmas shopping in Oz....in the days before the big day.
We'd get summer clothes, swimsuits, pool toys, hand-held gaming consoles are small and easily hidden....I would plan on trying to shop a bit once you get here. What do your kids want for Christmas? Anything in particular?
Also, what part of Oz are you visiting?