Dec/Jan is a great time to see Patagonia…but plan ahead…Argentineans also take their summer vacations then. Since the devaluation, most Argentines are not leaving the country for holiday, so places fill up fast. However, having said that, I was in the Patagonian Andes in Dec/Jan this past year and never had a problem finding a room. Also, flights to Buenos Aires are the most expensive in December (at least from the US, not sure about from Australia).

InterPatagonia.com is a great site for planning your trip.