In September of 2007 the Prime Minister met with the Prime Minister of Australia, and they actually signed an agreement to do an exchange. I believe the purpose of that was to share best practices and also for career development.
Yvan may have more information on why Australia was chosen, but it is a Westminster country. Therefore, within their system we may actually learn best practices that are somewhat more applicable to our type of government.
We also have quite a tradition in the Canadian government of having exchanges with the Australian government in other departments. It was probably just a natural affinity because of the type of system that we operate under.