Couple of websites that may whet your appetite and give you some ideas of what to do when you arrive. http://www.sydney.com & http://www.australia.com/index.aspx
As far as what you do on New Year's eve, I reckon wait until you get here and get a feel for some of the different things you can do or places to go. There will be plenty of choices.
If you want to do a New Year's Eve harbour cruise, remember it could be fairly expensive and you will probably need to get in early.
My suggestion would be to make sure you're somewhere near the harbour so you get a good view of the fireworks. There will be plenty of all day (and night) parties on the foreshores for you to join.
Christmas Day. Yes, Bondi is very popular but I still don't know why But everyone from Britain seems to end up there. Bondi is just south of the city but I reckon the northern beaches are better. Like Manly.
Fruit picking is popular for working holidays, as is bar work, waitress or something in the hospitality industry.
Maybe doing a bar course is worthwhile before you arrive. You never know, you might get a job on one of the resort islands off Queensland.
Quite a few people here on working holidays find all sorts of casual work in Sydney as well. There are always jobs going if you don't mind too much what you're doing. Process work like stuffing envelopes etc.
Some of the people that stayed on this year found work local to us as well.
It's the middle of winter here now but today is mild and sunny with a top of about 18 degrees expected (bit warmer than it should be).
Be prepared for the heat when you arrive. Christmas Day on a beach here is hot enough to fry the skin off your back. Literally!
I'll just feed you tidbits every now and again to keep you interested