ext (top level domain) other than the default results in NA output in the function geocode() in the ggmap library

Question

In the help for for ggmap::geocode I read 'ext: top level domain (e.g. "com", "co.nz"); helpful for non-US users'

I want to set the top level domain as Australia as most (but not all) of my addresses are Australian and because I see that many of the incorrect geocoded addresses refer to US addresses (default domain is set at 'com'). Using the front end of google, I often do get the correct address as my browser has the AU time zone (I guess this is why).

I tried setting ext as 'AU', 'au', 'com.au', but none of them work. They all return NA. Setting it back to 'com' works, setting it to 'co.nz' as in the help file also returns NA.

    ggmap::geocode(location = "Yamba Dr, Garran ACT 2605, # just an example, it's the Canberra hospital
                   output   = 'all', 
                   ext      = "au")
NA

Any ideas?