Simplify the following text for a Grade 6 native English-speaking person.
Original: After the arrival of General Douglas MacArthur in Australia during the Second World War , Bankstown Airport was established as a key strategic air force base to support the war effort . During this period the specially constructed bunker became an important Royal Australian Air Force headquarters from 1945 until its closure in 1947 .
Simplified: The Bankstown Bunker was a top secret Australian Air Force base that was once used during the Second World War .