According to the New York Times travel section, the months to book flights to Australia is April through August because the U.S. airlines offer the cheapest prices to fly.
In the ensuing months, the equinox will be slowly changing to welcome summer. Although it does not officially began until June, our summer is directly opposite to Australia. You have to remember that when we have our spring and summer, Australia has their winter, and when we have our winter, they have their summer.
So, what is an Australian winter like? Where are the best places to go for their winter?
April through September are the best winter months to travel when visiting the northern parts of Austrailia during the winter as the temperature is 8 degrees celcius or 46 degrees fahrenheit during the night, and 16 degrees celsius or 60 degrees fahrenheit during the day. Cold enough to keep the snow, and nice enough to not be too chilly.
Their summers are nice--80-to 86 degrees fahrenheit, but depending on where you go, such as the desert, you will encounter very hot weather, up to 100 degrees fahrenheit.
You can refer to http://en.wikipedia.org/wiki/Climate_of_Australia for more information about the weather in Australia.
Where to visit
The best places for a great ski season is in the mountains of New South Wales and Northeastern Victoria. Tasmania also gets a good amount of snow. Book your airline tickets for July through August as this is the peak of their winter. You will want to go to the Kosciuszko National Park in New South Wales for their ski resorts--Charlottes Pass, Thredbo, and Blue Cow. If you plan of visiting Victoria--Mt Bogong, Fall's Creek, Mt Hotham are the ski resorts there.
Sydney is also a great place to visit during the winter season. You can visit the Opera House, watch the whales as they travel north, walking tours, Yulefest in the Blue Mountains, and many other activities.
Cost of renting ski equipment
Don't bother bringing your skis. Not only would you have to pay an exhorbant fee for oversized baggage, you don't really want to chance getting them broken. You can bring goggles, ski clothes, helmet, and boots to cut down on cost. Current prices I have found are skis, boots, and poles for adults, for 7 days, will be approximately 105 AUS dollars. The current exchange for U.S. currency to the Australian dollar is not a big difference--1.03 to 1.00. That is an expensive rental.
If you book in April, you will get the best airline tickets through air-savings.com. If you plan on any tours while there, they can package it, and get you a really great discount to enjoy your vacation.