This is a cover I received back from the Ross Dependency the other day. I sent it down there around the end of November and it came back in fairly quick time considering the time of year. It was also wrapped in plastic so it came though the postal system nice and fresh.
Although the cover includes what appears to be a 50th anniversary cachet on it, it's likely this was a cachet produced for the 50th anniversary of the New Zealand Antarctic program which was back in 2006, and not the 50th anniversary of the Antarctic Treaty which was Dec of 2009 and commemorated on the stamp on this cover.
Mail hasn't been processed at the Scott Base on the continent since 1987. All mail to and from the base is handled by the Dependency post office in Christchurch New Zealand, hence the Christchurch cancel on this cover.