Here is one way to do it:
RewriteEngine On
RewriteRule ^us/(.*)$ http://au.domain.com/en/$1 [L,R=301]
Redirects:
http://au.domain.com/us/anything
to
http://au.domain.com/en/anything/
UPDATED
RewriteEngine On
RewriteRule http://au.domain.com/us/^(.*)$ http://au.domain.com/en/$1 [L,R=301]
Only http://au.domain.com/us/anything
will be redirected to
http://au.domain.com/en/anything/
OR
RewriteEngine On
RewriteCond %{HTTP_HOST} ^au.domain\.com$ [NC]
RewriteRule ^us/(.*)$ http://au.domain.com/en/$1 [L,R=301]
Hope this helps.