As Harm is driving away from the airport he is driving on the left side of the road as required by the road rules in Australia. When they come to what is known in America as a 'traffic circle,' Brumby refers to it as a 'circle,' (presumably expecting Harm to know that term). In Australia, they are called 'roundabouts' and are to be driven around in an clockwise direction.
"Boomerang: Part 1" is the 100th episode of the series.
When Bud and Harm arrive in Australia direct from the USA, they exit from the Qantas domestic terminal. This on the opposite side of the Airport from the International terminal with no direct access between the two, also being a domestic terminal. There are no Customs or Immigration facilities there, so it would be highly unlikely that relatively Junior Military Officers would be granted any form of VIP treatment that would enable them to bypass Customs and Immigration.
Steven Culp was supposed to make the trip to Australia but was unable due to prior commitment, Bellisario didn't want to cut the scenes, so a double was used for his scenes. Where Rabb and Webb are face to face it is Steven Culp.