Think about your post. The plane is not just for use by the PM....

Think about your post.
The plane is not just for use by the PM. He usually travels with several advisors. It is impractical for him to travel by commercial airlines, because of scheduling, security for him, and security for passengers in a commercial plane (if he travelled commercially). Discussions will be carried out en-route.
The plane will also be used by other government officials when travelling overseas.
Do you expect our PM to fly in a small learjet, or an Australian air force cargo plane?