I suggest you read the research/ FAQ's and decide where you want to go. Australia is a large place with different weather regions at xmas.
Once you have an approximate itinerary and budget people will be happy to assist with further questions.
" Need help to work out an itinerary to try and fit in the other must sees"
What are your other must sees?
Keep in mind December is the wet season in northern Australia, so at that time of year you are probably better to focus on the southern states. Having said that it is the busy season in the south with school holidays etc. so keep in mind you may need to book ahead for accomm etc.
As for NYE in Sydney - start looking, and booking if you can, now. And be prepared for high prices.
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